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/tests/arrays.sh

@@ -598,7 +598,7 @@ x=$(
 	foo[1]=(66)
 	typeset -p foo
 ) 2> /dev/null
-[[ $x == "$exp" ]] || err_exit 'setting element 1 to index fooay failed'
+[[ $x == "$exp" ]] || err_exit 'setting element 1 of indexed array foo failed'
 unset foo
 exp='typeset -a foo=((11 22) (x=3))'
 x=$(