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microtime.9: rewrite description, miscellaneous cleanup

- Remove the bintime interfaces.  They should not be used outside of
  the timecounting layer.  Unsure whether they warrant a manpage of
  their own.
- In the SYNOPSIS, change the variable names for timespec interfaces
  from "tv" to "ts".
- Document the new-ish "nsec" interfaces.
- Rewrite the DESCRIPTION.  Describe every clock completely in its own
  paragraph.  Enumerate all the interfaces in tables.  Explicitly state
  the output format for each interface in said tables.  Add new vocab
  ("hardware", "timestamp") to clarify the differences between the
  "get" and non-"get" interfaces.
- Add the CONTEXT, RETURN VALUES, and ERRORS sections.
- Cross-reference clock_settime(2), timeradd(3), and tc_init(9).

Lots of input from schwarze@.

ok jmc@ schwarze@

.\"	$OpenBSD: microtime.9,v 1.23 2022/12/28 15:46:39 cheloha Exp $
.\"     $NetBSD: microtime.9,v 1.2 1999/03/16 00:40:47 garbled Exp $
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.\" by Jeremy Cooper.
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.Dd $Mdocdate: December 28 2022 $
.Dt MICROTIME 9
.Os
.Sh NAME
.Nm microuptime ,
.Nm getmicrouptime ,
.Nm nanouptime ,
.Nm getnanouptime ,
.Nm nsecuptime ,
.Nm getnsecuptime ,
.Nm getuptime ,
.Nm nanoruntime ,
.Nm getnsecruntime ,
.Nm microtime ,
.Nm getmicrotime ,
.Nm nanotime ,
.Nm getnanotime ,
.Nm gettime ,
.Nm microboottime ,
.Nm nanoboottime
.Nd get the time
.Sh SYNOPSIS
.In sys/time.h
.Ft void
.Fo microuptime
.Fa "struct timeval *tv"
.Fc
.Ft void
.Fo getmicrouptime
.Fa "struct timeval *tv"
.Fc
.Ft void
.Fo nanouptime
.Fa "struct timespec *ts"
.Fc
.Ft void
.Fo getnanouptime
.Fa "struct timespec *ts"
.Fc
.Ft uint64_t
.Fo nsecuptime
.Fa "void"
.Fc
.Ft uint64_t
.Fo getnsecuptime
.Fa "void"
.Fc
.Ft time_t
.Fo getuptime
.Fa "void"
.Fc
.Ft void
.Fo nanoruntime
.Fa "struct timespec *ts"
.Fc
.Ft uint64_t
.Fo getnsecruntime
.Fa "void"
.Fc
.Ft void
.Fo microtime
.Fa "struct timeval *tv"
.Fc
.Ft void
.Fo getmicrotime
.Fa "struct timeval *tv"
.Fc
.Ft void
.Fo nanotime
.Fa "struct timespec *ts"
.Fc
.Ft void
.Fo getnanotime
.Fa "struct timespec *ts"
.Fc
.Ft time_t
.Fo gettime
.Fa "void"
.Fc
.Ft void
.Fo microboottime
.Fa "struct timeval *tv"
.Fc
.Ft void
.Fo nanoboottime
.Fa "struct timespec *ts"
.Fc
.Sh DESCRIPTION
The kernel has three clocks and a variety of interfaces for reading them.
.Pp
The
.Sy uptime
clock measures the time elapsed since the system booted.
It begins at zero and advances monotonically.
The uptime clock may be read with the following functions:
.Bl -column "getmicrouptimeX" "Output Format" "Source" -offset indent
.It Em Name            Ta Em Output Format    Ta Em Source
.It Fn microuptime     Ta Vt struct timeval   Ta hardware
.It Fn getmicrouptime  Ta Vt struct timeval   Ta timestamp
.It Fn nanouptime      Ta Vt struct timespec  Ta hardware
.It Fn getnanouptime   Ta Vt struct timespec  Ta timestamp
.It Fn nsecuptime      Ta Ft uint64_t         Ta hardware
.It Fn getnsecuptime   Ta Ft uint64_t         Ta timestamp
.It Fn getuptime       Ta Ft time_t           Ta timestamp
.El
.Pp
The
.Sy runtime
clock measures the time elapsed since the system booted,
less any time the system is suspended or hibernating.
It begins at zero and normally advances monotonically,
but pauses while the system is suspended or hibernating.
The runtime clock may be read with the following functions:
.Bl -column "getnsecruntimeX" "Output Format" "Source" -offset indent
.It Em Name            Ta Em Output Format    Ta Em Source
.It Fn nanoruntime     Ta Vt struct timespec  Ta hardware
.It Fn getnsecruntime  Ta Ft uint64_t         Ta timestamp
.El
.Pp
The
.Sy UTC
clock measures the time elapsed since Jan 1 1970 00:00:00
.Pq the Unix Epoch .
The clock normally advances monotonically,
but jumps when a process calls
.Xr clock_settime 2
or
.Xr settimeofday 2 .
The UTC clock may be read with the following functions:
.Bl -column "getmicrotimeX" "Output Format" "Source" -offset indent
.It Em Name            Ta Em Output Format    Ta Em Source
.It Fn microtime       Ta Vt struct timeval   Ta hardware
.It Fn getmicrotime    Ta Vt struct timeval   Ta timestamp
.It Fn nanotime        Ta Vt struct timespec  Ta hardware
.It Fn getnanotime     Ta Vt struct timespec  Ta timestamp
.It Fn gettime         Ta Ft time_t           Ta timestamp
.El
.Pp
The kernel also maintains a
.Sy boot timestamp .
It is the moment on the UTC clock when the system booted.
The timestamp jumps when a process calls
.Xr clock_settime 2
or
.Xr settimeofday 2 .
The boot timestamp may be read with the following functions:
.Bl -column "microboottimeX" "Output Format" "Source" -offset indent
.It Em Name            Ta Em Output Format    Ta Em Source
.It Fn microboottime   Ta Vt struct timeval   Ta timestamp
.It Fn nanoboottime    Ta Vt struct timespec  Ta timestamp
.El
.Pp
Functions that source from the
.Em hardware
provide the most precise result possible.
Functions that source from a
.Em timestamp
provide a far less precise result,
but do so very quickly.
On most platforms,
timestamps are updated approximately 100 times per second.
.Sh CONTEXT
These functions may be called during autoconf,
from process context,
or from any interrupt context.
.Sh RETURN VALUES
.Fn nsecuptime ,
.Fn getnsecuptime ,
and
.Fn getnsecruntime
return a count of nanoseconds.
.Pp
.Fn getuptime
and
.Fn gettime
return a count of seconds.
.Sh ERRORS
These functions are always successful,
and no return value is reserved to indicate an error.
.Sh CODE REFERENCES
.Pa sys/kern/kern_tc.c
.Sh SEE ALSO
.Xr clock_settime 2 ,
.Xr settimeofday 2 ,
.Xr timeradd 3 ,
.Xr hardclock 9 ,
.Xr hz 9 ,
.Xr inittodr 9 ,
.Xr tc_init 9