The enclosed patch makes clone try to acquire a controlling tty. This
makes clone more useful because it enables job control in cloned
sessions.

The patch applies on current cvs head.

Notes about the patch:

 - We don't call TIOCNOTTY if HAVE_SETSID: setsid() automatically
   detaches from the current controlling tty.

 - The logic for dup'ing the newly opened tty is changed for a better
   one.

 - Once the dup'ing of the newly opened tty is done, we try to acquire
   the newly opened tty as a controlling tty. We print a message if
   this fails.

 - We have to reset mypgrp to zero so that init_io() can get its job
   done correctly.

Notes about controlling ttys:

 - Due to the way job control works, it is impossible for a process to
   acquire a controlling tty if another process already acquired it
   since:

     * POSIX mandates that if a session leader dies, all the processes
       in the session loose their controlling tty.

     * Most terminal emulator programs (including screen, but see note
       about screen) acquire a controlling tty before exec'ing their
       target process.

 - This means that the following won't work (the cloned zsh will run,
   but will not have a controlling tty (-> no job control)):

     * Open a new xterm.

     * Run tty in xterm.

     * Run in the xterm:

         exec zsh -c 'trap "" INT QUIT TSTP; while :; do sleep 100000; done'

     * Do a clone in the xterm's tty.

 - However this will work (the cloned zsh will have a controlling tty
   and job control will work):

     * clone /dev/tty<n> or clone /dev/vc/<n> on linux

     * zsh will be able to open cloned shells with job control in a
       screen session as long as the screen "setsid off" command is in
       effect in the screen:

        + Run screen. You're now in screen #0.

        + Enter "setsid off": this is done by typing in:

            ^A : setsid off <RETURN>

        + Create a new screen. You're now in screen #1.

        + The shell in screen 1 has job control disabled (it has no
          controlling tty).
 
        + In screen #1, run:

            tty && exec sleep 1000000

        + Switch back to screen #0: ^A 0

        + Run clone on the tty that was printed in screen #1.

        + Switch back to screen #1: ^A 1

        + You now have a cloned zsh with job control active.

Tested to work OK on Linux (with TIOCNOTTY/TIOCSCTTY controlling tty
acquiring method) and Solaris (with SvR4's O_NOCTTY controlling tty
acquiring method).

Phil.

Changelog entry:

2003-07-22  Philippe Troin  <phil@fifi.org>

	* Src/Modules/clone.c (bin_clone): Try to acquire a controlling
	tty when possible. Report failures. Cleanup.