replace <sys/types.h> with <unistd.h> and fix type of brk();

PRs 2938 and 3132.
also added xref to mmap(2).

lib/libc/sys/brk.2

@@ -1,4 +1,4 @@
-.\"	$NetBSD: brk.2,v 1.7 1995/02/27 12:31:57 cgd Exp $
+.\"	$NetBSD: brk.2,v 1.8 1997/01/23 05:32:52 mikel Exp $
 .\"
 .\" Copyright (c) 1980, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -41,8 +41,8 @@
 .Nm sbrk
 .Nd change data segment size
 .Sh SYNOPSIS
-.Fd #include <sys/types.h>
-.Ft int
+.Fd #include <unistd.h>
+.Ft char *
 .Fn brk "const char *addr"
 .Ft char *
 .Fn sbrk "int incr"
@@ -51,6 +51,7 @@
 The brk and sbrk functions are historical curiosities
 left over from earlier days before the advent of virtual memory management.
 .Ef
+.Pp
 The
 .Fn brk
 function
@@ -134,6 +135,7 @@ to support the expansion.
 .Sh SEE ALSO
 .Xr execve 2 ,
 .Xr getrlimit 2 ,
+.Xr mmap 2 ,
 .Xr malloc 3 ,
 .Xr end 3
 .Sh BUGS