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/include/defs.h

@@ -91,8 +91,8 @@ extern char*	sh_setenviron(const char*);
 #define SH_CMDLIB_DIR	"/opt/ast/bin"
 #endif
 
-#define SH_ID			"ksh"	/* ksh id */
-#define SH_STD			"sh"	/* standard sh id */
+#define SH_ID			"ksh"	/* ksh ID */
+#define SH_STD			"sh"	/* standard sh ID */
 
 /* defines for sh_type() */
 

src/cmd/ksh93/include/jobs.h

@@ -60,7 +60,7 @@ struct process
 	struct process *p_nxtjob;	/* next job structure */
 	struct process *p_nxtproc;	/* next process in current job */
 	int		*p_exitval;	/* place to store the exitval */
-	pid_t		p_pid;		/* process id */
+	pid_t		p_pid;		/* process ID */
 	pid_t		p_pgrp;		/* process group */
 	pid_t		p_fgrp;		/* process group when stopped */
 	short		p_job;		/* job number of process */
@@ -78,11 +78,11 @@ struct jobs
 {
 	struct process	*pwlist;	/* head of process list */
 	int		*exitval;	/* pipe exit values */
-	pid_t		curpgid;	/* current process gid id */
+	pid_t		curpgid;	/* current process GID */
 	pid_t		parent;		/* set by fork() */
-	pid_t		mypid;		/* process id of shell */
-	pid_t		mypgid;		/* process group id of shell */
-	pid_t		mytgid;		/* terminal group id of shell */
+	pid_t		mypid;		/* process ID of shell */
+	pid_t		mypgid;		/* process group ID of shell */
+	pid_t		mytgid;		/* terminal group ID of shell */
 	int		curjobid;
 	unsigned int	in_critical;	/* >0 => in critical region */
 	int		savesig;	/* active signal */

src/cmd/ksh93/include/shlex.h

@@ -99,7 +99,7 @@ typedef struct  _shlex_
 	unsigned long	unknown;	/* <unknown> entity number */
 	off_t		kiabegin;	/* offset of first entry */
 	char		*scriptname;	/* name of script file */
-	Dt_t		*entity_tree;	/* for entity ids */
+	Dt_t		*entity_tree;	/* for entity IDs */
 #endif /* SHOPT_KIA */
 	/* The following two struct members are considered private to lex.c */
 	struct _shlex_pvt_lexdata_  lexd;
@@ -167,7 +167,7 @@ typedef struct  _shlex_
 #define	SH_ASSIGN	020
 #define	SH_FUNDEF	040
 #define SH_ARRAY	0100
-#define SH_SEMI		0200	/* semi-colon after NL ok */
+#define SH_SEMI		0200	/* semicolon after NL ok */
 
 #define SH_COMPASSIGN	010	/* allow compound assignments only */