Add a man page for part of the PRNG API.

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+.\"
+.\" Copyright (c) 2000
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" All rights reserved.
+.\"
+.\" 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.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``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 DEVELOPERS 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.
+.\"
+.\" $FreeBSD$
+.\" "
+.Dd September 25, 2000
+.Os
+.Dt RANDOM 9
+.Sh NAME
+.Nm srandom,
+.Nm random,
+.Nm arc4random,
+.Nm read_random
+.Nd supply pseudo-random numbers.
+.Sh SYNOPSIS
+.Fd #include <sys/libkern.h>
+.Ft void
+.Fn srandom "u_long seed"
+.Ft u_long
+.Fn random "void"
+.Ft u_int32_t
+.Fn arc4random "void"
+.Pp
+.Fd #include <sys/random.h>
+.Ft u_int
+.Fn read_random "void *buffer" "u_int count"
+.Sh DESCRIPTION
+The
+.Fn random
+function will by default produce a sequence of numbers that can be duplicated
+by calling
+.Fn srandom
+with 
+.Ql 1
+as the
+.Ar seed .
+The
+.Fn srandom
+function may be called with any arbitrary
+.Ar seed
+value to get slightly more unpredictable numbers. 
+It is important to remember that the
+.Fn random
+function is entirely predictable, and is therefore not of use where
+knowledge of the sequence of numbers may be of benefit to an attacker.
+.Pp
+The
+.Fn arc4random
+function will return very good quality random numbers, slightly better
+suited for security-related purposes.
+The random numbers from
+.Fn arc4random
+are seeded from the entropy device if it is available.
+.Pp
+The
+.Fn read_random
+function is used to return entropy directly from the entropy device
+if it has been loaded. If the entropy device is not loaded, then
+the
+.Ar buffer
+is filled with output generated by
+.Fn random .
+The
+.Ar buffer
+is filled with no more than
+.Ar count
+bytes. It is advised that
+.Fn read_random
+is not used; instead use
+.Fn arc4random .
+.Pp
+All the bits generated by
+.Fn random ,
+.Fn arc4random
+and
+.Fn read_random
+are usable.  For example,
+.Sq Li random()&01
+will produce a random binary value.
+.Sh RETURN VALUES
+The
+.Fn random
+function
+uses a non-linear additive feedback random number generator employing a
+default table of size 31 long integers to return successive pseudo-random
+numbers in the range from 0 to
+.if t 2\u\s731\s10\d\(mi1.
+.if n (2**31)\(mi1.
+The period of this random number generator is very large, approximately
+.if t 16\(mu(2\u\s731\s10\d\(mi1).
+.if n 16*((2**31)\(mi1).
+.Pp
+The
+.Fn arc4random
+function
+uses the RC4 algorithm to generate successive pseudo-random
+numbers in the range from 0 to
+.if t 2\u\s732\s10\d\(mi1.
+.if n (2**32)\(mi1.
+.Pp
+The
+.Fn read_random
+function returns the number of bytes placed in
+.Ar buffer .
+.Pp
+.Sh AUTHORS
+.An Dan Moschuk
+wrote
+.Fn arc4random .
+.An Mark R V Murray
+wrote
+.Fn read_random .