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/path.c

@@ -556,7 +556,7 @@ char	*path_basename(register const char *name)
 
 char *path_fullname(const char *name)
 {
-	int len=strlen(name)+1,dirlen=0;
+	size_t len=strlen(name)+1,dirlen=0;
 	char *path,*pwd;
 	if(*name!='/')
 	{
@@ -1356,7 +1356,7 @@ static noreturn void exscript(register char *path,register char *argv[],char **e
 		if((euserid=geteuid()) != sh.userid)
 		{
 			strncpy(name+9,fmtbase((long)sh.current_pid,10,0),sizeof(name)-10);
-			/* create a suid open file with owner equal effective uid */
+			/* create an SUID open file with owner equal to effective UID */
 			if((n=open(name,O_CREAT|O_TRUNC|O_WRONLY,S_ISUID|S_IXUSR)) < 0)
 				goto fail;
 			unlink(name);