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.

ANNOUNCE

@@ -190,6 +190,12 @@ New shell language features:
   command, which never worked and crashed the shell. See the manual for
   details on the new method.
 
+- A new ${.sh.pid} variable has been added with similar functionality to
+  Bash's $BASHPID variable. It is set to the current shell's PID, unlike
+  $$ (which is set to the main shell's PID). In virtual subshells
+  ${.sh.pid} is not changed from its previous value, while in forked
+  subshells ${.sh.pid} is set to the subshell's process ID.
+
 New features in built-in commands:
 
 - Usage error messages now show the --help/--man self-documentation options.