Fix various typos, man page issues and improve the documentation (#415)

This commit makes various different improvements to the documentation:
- sh.1: Backported (with changes) mandoc warning fixes from ksh2020
  for the ksh93(1) man page: <https://github.com/att/ast/pull/1406>
- Removed unnecessary spaces at the end of lines to fix a few other
  mandoc warnings.
- Fixed various typos and capitalization errors in the documentation.
- ANNOUNCE: Document the addition of the ${.sh.pid} variable
  (re: 9de65210).
- libast/man/str*: Update the man pages for the libast str* functions
  to improve how accurately each function is described.
- ksh93/README: Update regression test/compatibility notes to include
  OpenBSD 7.0, FreeBSD 13.0 and WSL running Ubuntu 20.04.
- Change a few places to store the return value from strlen in a
  size_t variable rather than signed int.
- comp/setlocale.c: To avoid confusion of two separate variables named
  lang, the function local variable has been renamed to langidx.

src/lib/libast/sfio/vthread.h

@@ -126,7 +126,7 @@ struct _vtmutex_s
 /* structure for states of thread */
 struct _vthread_s
 {	_vtself_t	self;		/* self-handle		*/
-	_vtid_t		id;		/* thread id		*/
+	_vtid_t		id;		/* thread ID		*/
 	_vtattr_t	attrs;		/* attributes		*/
 	size_t		stack;		/* stack size		*/
 	int		state;		/* execution state	*/