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/misc/conformance.c

@@ -29,7 +29,7 @@ static char**		ids;
 static const char*	dflt[] = { "ast", "standard", 0 };
 
 /*
- * initialize the conformance() id list
+ * initialize the conformance() ID list
  */
 
 static char**
@@ -97,8 +97,8 @@ initconformance(void)
 }
 
 /*
- * return conformance id if s size n is in conformance
- * prefix match of s on the conformance id table
+ * return conformance ID if s size n is in conformance
+ * prefix match of s on the conformance ID table
  * s==0 => "standard"
  */