Accumulated fixes for minor issues (#442)

This commit applies various accumulated bugfixes:

- Applied some fixes for compiler warnings based off of the
  following pull requests (with whitespace changes excluded to
  avoid inflating the size of the diff):

  https://github.com/att/ast/pull/281
  https://github.com/att/ast/pull/283
  https://github.com/att/ast/pull/303
  https://github.com/att/ast/pull/304

- clone_type(): Two separate variables in this function share the
  same name. A bugfix from ksh93v- 2013-05-24 was backported to
  avoid conflict issues.

- Backported a minor error message improvement from ksh2020 for
  when the user attempts to run too many coprocesses.

- Backported a ksh2020 bugfix for a file descriptor leak:
  58bc8b56

- Backported ksh2020 bugfixes for unused variable and pointless
  assignment lint warnings:
  47650fe0
  df209c0d
  5e417b00

- Applied a few minor improvements to libast from graphviz:
  e7c03541
  969a7cde

- dtmemory(): Mark unused parameters with NOT_USED(). Based on:
  6ac3ad99

- Applied a few documentation/comment tweaks for the NEWS file,
  printf -v and spawnveg.

- Added a missing regression test for using the rm builtin's -f
  option without additional arguments (this was fixed in
  ksh93u+ 2012-02-14).

src/lib/libast/sfio/sfmode.c

@@ -2,7 +2,7 @@
 *                                                                      *
 *               This software is part of the ast package               *
 *          Copyright (c) 1985-2012 AT&T Intellectual Property          *
-*          Copyright (c) 2020-2021 Contributors to ksh 93u+m           *
+*          Copyright (c) 2020-2022 Contributors to ksh 93u+m           *
 *                      and is licensed under the                       *
 *                 Eclipse Public License, Version 1.0                  *
 *                    by AT&T Intellectual Property                     *
@@ -59,7 +59,9 @@ static char*	Version = "\n@(#)$Id: sfio (AT&T Labs - Research) 2009-09-15 $\0\n"
 #include		<signal.h>
 typedef void(*		Sfsignal_f)(int);
 #endif
+#ifdef SIGPIPE
 static int		_Sfsigp = 0; /* # of streams needing SIGPIPE protection */ 
+#endif
 
 /* done at exiting time */
 static void _sfcleanup(void)