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/sh.1

@@ -803,10 +803,6 @@ but can be unset or redefined:
 .B "nameref=\(fmtypeset \-n\(fm"
 .TP
 .B "r=\(fmhist \-s\(fm"
-.TP
-.B "stop=\(fmkill \-s \s-1STOP\s+1\(fm"
-.TP
-.B "suspend=\(fmkill \-s \s-1STOP\s+1 $$\(fm"
 .PD
 .RE
 .SS Tilde Substitution.
@@ -7133,6 +7129,28 @@ Same as
 .BR \|.\^ ,
 except it is not treated as a special built-in command.
 .TP
+\f3stop\fP \f2job\^\fP .\|.\|.
+Sends a
+.B SIGSTOP
+signal to one or more processes specified by
+.IR job ,
+suspending them until they receive
+.BR SIGCONT .
+The same as
+.BR kill\ -s\ STOP .
+.TP
+\f3suspend\fP
+Sends a
+.B SIGSTOP
+signal to the main shell process, suspending the script
+or child shell session until it receives
+.B SIGCONT
+(for instance, when typing
+.B fg
+in the parent shell). Equivalent to
+.BR kill\ -s\ STOP\ "$$" ,
+except that it accepts no operands and refuses to suspend a login shell.
+.TP
 \f3times\fP
 Displays the accumulated user and system CPU times, one line with the times
 used by the shell and another with those used by all of the shell's child