scripts: fix dumping execution backtrace

Dumping the backtrace has been broken since changeset #652e56d6d35a:
scripts: execute each steps in a subshell

We can spawn sub-sub-shells in some cases.

The way the fault handler works is to dump the backtrace, but to avoid
printing it once for every sub-shell (which could get quite confusing),
it simply exits when it detects that it is being run in a sub-shell,
leaving to the top-level shell the work to dump the backtrace.

Because each step is executed in its own sub-shell, the variable arrays
that contain the step name, the source file and line number, are lost
when exiting the per-step sub-shell.

Hence, the backtrace is currently limited to printing only the top-level
main procedure of the shell.

Fix this thus:
- when dumping the bckatraces for the steps & the functions, remember
it was dumped, and only dump it if it was not already dumped
- at the top-level shell, print the hints

Also, rename the top-level step label.

Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 4193d6e6a17430a177fa88c287879c2c35e319f3)

 only fork one to assist in stop-start-daemon assumptions about daemon behavior

 http://bugs.gentoo.org/190785

 diff -durN glibc-2.10.1.orig/nscd/nscd.c glibc-2.10.1/nscd/nscd.c

 --- glibc-2.10.1.orig/nscd/nscd.c	2009-02-06 21:10:27.000000000 +0100

 +++ glibc-2.10.1/nscd/nscd.c	2009-11-13 00:50:24.000000000 +0100

 @@ -182,6 +182,9 @@

        if (pid != 0)

  	exit (0);

 +       if (write_pid (_PATH_NSCDPID) < 0)

 +	dbg_log ("%s: %s", _PATH_NSCDPID, strerror (errno));

 +

        int nullfd = open (_PATH_DEVNULL, O_RDWR);

        if (nullfd != -1)

  	{

 @@ -231,12 +234,6 @@

  	for (i = min_close_fd; i < getdtablesize (); i++)

  	  close (i);

 -      pid = fork ();

 -      if (pid == -1)

 -	error (EXIT_FAILURE, errno, _("cannot fork"));

 -      if (pid != 0)

 -	exit (0);

 -

        setsid ();

        if (chdir ("/") != 0)

 @@ -245,9 +242,6 @@

        openlog ("nscd", LOG_CONS | LOG_ODELAY, LOG_DAEMON);

 -      if (write_pid (_PATH_NSCDPID) < 0)

 -        dbg_log ("%s: %s", _PATH_NSCDPID, strerror (errno));

 -

        if (!init_logfile ())

  	dbg_log (_("Could not create log file"));