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)

Original patch from http://gmplib.org/patches/mpf_set_str.c.diff

Re-diffed to match crosstool-NG conventions.

diff -dur gmp-4.2.2.orig/mpf/set_str.c gmp-4.2.2/mpf/set_str.c

--- gmp-4.2.2.orig/mpf/set_str.c	2007-08-30 20:31:40.000000000 +0200

+++ gmp-4.2.2/mpf/set_str.c	2008-01-28 23:05:29.000000000 +0100

@@ -271,8 +271,29 @@

       }

     if (expptr != 0)

-      /* FIXME: Should do some error checking here.  */

-      exp_in_base = strtol (expptr, (char **) 0, exp_base);

+      {

+	/* Scan and convert the exponent, in base exp_base.  */

+	long dig, neg = -(long) ('-' == expptr[0]);

+	expptr -= neg;			/* conditional increment */

+	c = (unsigned char) *expptr++;

+	dig = digit_value[c];

+	if (dig >= exp_base)

+	  {

+	    TMP_FREE;

+	    return -1;

+	  }

+	exp_in_base = dig;

+	c = (unsigned char) *expptr++;

+	dig = digit_value[c];

+	while (dig < exp_base)

+	  {

+	    exp_in_base = exp_in_base * exp_base;

+	    exp_in_base += dig;

+	    c = (unsigned char) *expptr++;

+	    dig = digit_value[c];

+	  }

+	exp_in_base = (exp_in_base ^ neg) - neg; /* conditional negation */

+      }

     else

       exp_in_base = 0;

     if (dotpos != 0)