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)

 diff --git a/mpz/remove.c b/mpz/remove.c

 --- a/mpz/remove.c

 +++ b/mpz/remove.c

 @@ -23,7 +23,7 @@

  unsigned long int

  mpz_remove (mpz_ptr dest, mpz_srcptr src, mpz_srcptr f)

  {

 -  mpz_t fpow[40];		/* inexhaustible...until year 2020 or so */

 +  mpz_t fpow[GMP_LIMB_BITS];		/* Really MP_SIZE_T_BITS */

    mpz_t x, rem;

    unsigned long int pwr;

    int p;

 @@ -69,7 +69,7 @@

        mpz_set (dest, x);

      }

 -  pwr = (1 << p) - 1;

 +  pwr = (1L << p) - 1;

    mpz_clear (fpow[p]);

 diff --git a/mpz/remove.c b/mpz/remove.c

 --- a/mpz/remove.c

 +++ b/mpz/remove.c

 @@ -80,7 +80,7 @@

        mpz_tdiv_qr (x, rem, dest, fpow[p]);

        if (SIZ (rem) == 0)

  	{

 -	  pwr += 1 << p;

 +	  pwr += 1L << p;

  	  mpz_set (dest, x);

  	}

        mpz_clear (fpow[p]);