Import some documentation from NetBSD, after checking that details match.

hz.9 and hzto.9 from scratch

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+.\"     $NetBSD: microtime.9,v 1.2 1999/03/16 00:40:47 garbled Exp $
+.\"
+.\" Copyright (c) 1998 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" This code is derived from software contributed to The NetBSD Foundation
+.\" by Jeremy Cooper.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"        This product includes software developed by the NetBSD
+.\"        Foundation, Inc. and its contributors.
+.\" 4. Neither the name of The NetBSD Foundation nor the names of its
+.\"    contributors may be used to endorse or promote products derived
+.\"    from this software without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd Sep 14, 1998
+.Dt MICROTIME 9
+.Os
+.Sh NAME
+.Nm microtime
+.Nd realtime system clock
+.Sh SYNOPSIS
+.Fd #include <sys/time.h>
+.Ft void
+.Fo "microtime"
+.Fa "struct timeval *tv"
+.Fc
+.Sh DESCRIPTION
+.Fn microtime
+returns the current value of the system realtime clock in the structure
+pointed to by the argument
+.Fa tv .
+The system realtime clock is guaranteed to be monotonically increasing
+at all times.
+As such,
+all calls to
+.Fn microtime
+are guaranteed to return a system time greater than or equal to the
+system time returned in any previous calls.
+.Sh SEE ALSO
+.Xr hz 9 ,
+.Xr hardclock 9 ,
+.Xr inittodr 9 ,
+.Xr time 9 ,
+.Xr settimeofday 9
+.Sh CODE REFERENCES
+The implementation of the
+.Fn microtime
+function is machine dependent,
+hence its location in the source code tree varies from architecture to
+architecture.
+.Sh BUGS
+Despite the guarantee that the system realtime clock will always be
+monotonically increasing,
+it is always possible for the system clock to be manually reset by the
+system administrator to any date.