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/INIT/RELEASE

@@ -528,7 +528,7 @@
 02-10-18 probe.win32: update for mingw
 	 make.probe: add bash workaround to SHELLMAGIC test
 	 package.sh: work around yet another Cygwin hassle
-02-10-17 iffe.sh: short circuit id check for *[-+/\\]*
+02-10-17 iffe.sh: short circuit ID check for *[-+/\\]*
 02-10-08 regress.sh: unset FIGNORE to avoid rm . errors
 	 package.sh: unset FIGNORE to avoid rm . errors
 	 package.sh: $CC must at least compile and executable hello world
@@ -580,7 +580,7 @@
 02-03-24 mamake.c: all shell actions wrapped with -c to avoid #! problems
 02-03-23 package.sh: recover $PACKAGEROOT/bin/package if not in INIT package
 	 package.sh: precheck $CC, not `cc'
-	 package.sh: fix install to use pax -ps to preserve set-uid/gid
+	 package.sh: fix install to use pax -ps to preserve setuid/setgid
 	 package.sh: fix install to use list.installed for existing files only
 02-03-17 package.sh: fix PAX initialization that was sometimes omitted for read
 	 package.sh: fix update delta sync fetch
@@ -906,7 +906,7 @@
 96-10-11 iffe: *.iffe and *.iff for iffe src files
 	 hostinfo: tighten sgi.mips CPU type check
 96-10-01 C.probe: add probe_libdir to catch alpha /usr/shlib
-96-09-17 iffe: fix typ bug that failed for single id types!
+96-09-17 iffe: fix typ bug that failed for single ID types!
 96-08-31 hostinfo: handle recent SGI hinv CPU changes
 96-07-17 make sure sizeof(long xxx)>sizeof(xxx) for typ long.xxx
 96-05-09 C.probe: drop multiple / in stdinclude

src/cmd/INIT/mktest.sh

@@ -43,7 +43,7 @@ case $(getopts '[-][123:xyz]' opt --xyz 2>/dev/null; echo 0$opt) in
     template commands in the \aunit\a.\brt\b file. The generated test
     script writes temporary output to '$PREFIX$'\aunit\a.tmp and compares
     it to the expected output in '$PREFIX$'\aunit\a.out. Run the test
-    script with the \b--accept\b option to (re)generate the
+    script with the \b--accept\b option to (re)generate
     '$PREFIX$'\aunit\a.out.]
 [s:style?The script style:]:[style:='$STYLE$']
     {

src/cmd/INIT/regress.sh

@@ -128,7 +128,7 @@ unit [ command [ arg ... ] ]
             the files.]
         [+INFO \b\adescription\a?\adescription\a is printed on the
             standard error.]
-        [+INITIALIZE?Called by \bregress\b to initialize a each
+        [+INITIALIZE?Called by \bregress\b to initialize each
             \bTEST\b group.]
         [+INPUT \b[ \b-e\b \afilter\a ]] [ \b-n\b ]] \afile\a | - \adata\a ...?The
 	    standard input is set to either the contents of \afile\a