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/edit/emacs.c

@@ -159,7 +159,7 @@ characters in the middle of the line.
 
 typedef enum
 {
-	FIRST,		/* First time thru for logical line, prompt on screen */
+	FIRST,		/* First time through for logical line, prompt on screen */
 	REFRESH,	/* Redraw entire screen */
 	APPEND,		/* Append char before cursor to screen */
 	UPDATE,		/* Update the screen as need be */
@@ -1509,8 +1509,8 @@ static void draw(register Emacs_t *ep,Draw_t option)
 	}
 			
 	/*********************
-	 Is the range of screen[0] thru screen[w_size] up-to-date
-	 with nscreen[offset] thru nscreen[offset+w_size] ?
+	 Is the range of screen[0] through screen[w_size] up-to-date
+	 with nscreen[offset] through nscreen[offset+w_size] ?
 	 If not, update as need be.
 	***********************/
 	

src/cmd/ksh93/edit/vi.c

@@ -1157,7 +1157,7 @@ static void del_line(register Vi_t *vp, int mode)
 
 /*{	DELMOTION( motion, mode )
  *
- *	Delete thru motion.
+ *	Delete through motion.
  *
  *	mode	= 'd', save deleted characters, delete
  *		= 'c', do not save characters, change
@@ -2699,7 +2699,7 @@ deleol:
 		c = '$';
 		goto yankeol;
 
-	case 'y':		/** yank thru motion **/
+	case 'y':		/** yank through motion **/
 		if( mode )
 			c = vp->lastmotion;
 		else