Make 'stop' and 'suspend' regular built-ins

The 'stop' and 'suspend' default aliases are now converted into
regular built-in commands so that 'unalias -a' does not remove
them, 'suspend' can do some sanity checks, and something like
	cmd=stop; $cmd $!
will now work.

src/cmd/ksh93/bltins/trap.c:
- b_kill(): Incorporate 'stop' functionality, which is simply
  setting the same flag and variable as '-s STOP' would have done.
- b_suspend(): Add simple builtin function that sends SIGSTOP to
  the main shell. Check for no operands, and refuse to suspend a
  login shell (which would leave the user stuck with no way out).
  Also check that 'kill' succeeds; if we're in an asynchronous
  subshell, it is possible the main shell no longer exists.

src/cmd/ksh93/data/aliases.c:
- Remove "stop" and "suspend" default aliases. (Why were these
  conditional upon SIGTSTP when they actually issued SIGSTOP?)

src/cmd/ksh93/include/builtins.h,
src/cmd/ksh93/data/builtins.c,
src/cmd/ksh93/data/msg.c:
- Add declarations of "stop" and "suspend" regular built-ins.
- Add option strings (AST manual/--man pages) for them.
- Add e_toomanyops ("too many operands") reusable error message for
  b_suspend(). Other new commands may want this at some point.

src/cmd/ksh93/sh.1:
- Remove "stop" and "suspend" default aliases.
- Document "stop" and "suspend" regular built-in commands.

src/cmd/ksh93/data/aliases.c

@@ -37,10 +37,6 @@ const struct shtable2 shtab_aliases[] =
 	"integer",	NV_NOFREE|BLT_DCL,	"typeset -li",
 	"nameref",	NV_NOFREE|BLT_DCL,	"typeset -n",
 	"r",		NV_NOFREE,		"hist -s",
-#ifdef SIGTSTP
-	"stop",		NV_NOFREE,		"kill -s STOP",
-	"suspend", 	NV_NOFREE,		"kill -s STOP $$",
-#endif /*SIGTSTP */
 	"",		0,			(char*)0
 };
 

src/cmd/ksh93/data/builtins.c

@@ -109,6 +109,10 @@ const struct shtable3 shtab_builtins[] =
 	"/bin/kill",	NV_BLTIN|BLT_ENV,		bltin(kill),
 #   endif	/* SIGTSTP */
 	"jobs",		NV_BLTIN|BLT_ENV,		bltin(jobs),
+#   ifdef SIGSTOP
+	"stop",		NV_BLTIN|BLT_ENV,		bltin(kill),
+	"suspend", 	NV_BLTIN|BLT_ENV,		bltin(suspend),
+#   endif	/* SIGSTOP */
 #endif	/* JOBS */
 	"false",	NV_BLTIN|BLT_ENV,		bltin(false),
 	"getopts",	NV_BLTIN|BLT_ENV,		bltin(getopts),
@@ -1060,6 +1064,42 @@ _JOB_
 "[+SEE ALSO?\bps\b(1), \bjobs\b(1), \bkill\b(2), \bsignal\b(2)]"
 ;
 
+#if defined(JOBS) && defined(SIGSTOP)
+const char sh_optstop[] =
+"[-1c?\n@(#)$Id: stop (ksh93) 2020-06-22 $\n]"
+"[+NAME?stop - suspend a process]"
+"[+DESCRIPTION?\bstop\b sends a \bSIGSTOP\b signal to one or more processes "
+	"specified by \ajob\a, suspending them until they receive \bSIGCONT\b.]"
+_JOB_
+"\n"
+"\njob ...\n"
+"\n"
+"[+EXIT STATUS?]{"
+	"[+0?At least one matching process was found for each \ajob\a "
+	"operand, and \bSIGSTOP\b was successfully sent to at least one "
+	"matching process.]"
+	"[+>0?An error occurred.]"
+"}"
+"[+SEE ALSO?\bkill\b(1)]"
+;
+
+const char sh_optsuspend[] =
+"[-1c?\n@(#)$Id: suspend (ksh93) 2020-06-22 $\n]"
+"[+NAME?suspend - stop the shell]"
+"[+DESCRIPTION?\bsuspend\b sends a \bSIGSTOP\b signal to the main shell "
+	"process, suspending the script or child shell session until it "
+	"receives \bSIGCONT\b (for instance, when typing \bfg\b in the "
+	"parent shell).]"
+"[+?\bsuspend\b is equivalent to \bkill -s STOP \"$$\"\b, except that "
+	"it accepts no operands and refuses to suspend a login shell.]"
+"[+EXIT STATUS?]{"
+	"[+0?The shell was successfully suspended and continued.]"
+	"[+>0?An error occurred.]"
+"}"
+"[+SEE ALSO?\bkill\b(1)]"
+;
+#endif /* defined(JOBS) && defined(SIGSTOP) */
+
 const char sh_optlet[]	=
 "[-1c?@(#)$Id: let (AT&T Research) 2000-04-02 $\n]"
 USAGE_LICENSE

src/cmd/ksh93/data/msg.c

@@ -53,6 +53,7 @@ const char e_option[]		= "%s: bad option(s)";
 const char e_toomany[]		= "open file limit exceeded";
 const char e_argtype[]		= "invalid argument of type %c";
 const char e_oneoperand[]	= "one operand expected";
+const char e_toomanyops[]	= "too many operands";
 const char e_formspec[]		= "%c: unknown format specifier";
 const char e_badregexp[]	= "%s: invalid regular expression";
 const char e_number[]		= "%s: bad number";