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/cmd/ksh93/sh/subshell.c

@@ -76,7 +76,7 @@ static struct subshell
 	Dt_t		*strack;/* tracked alias scope for subshell */
 	Pathcomp_t	*pathlist; /* for PATH variable */
 	Shopt_t		options;/* save shell options */
-	pid_t		subpid;	/* child process id */
+	pid_t		subpid;	/* child process ID */
 	Sfio_t*		saveout;/* saved standard output */
 	char		*pwd;	/* present working directory */
 	void		*jobs;	/* save job info */