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)

 --- glibc-2.5/sysdeps/unix/sysv/linux/alpha/sigsuspend.S.orig	2006-10-22 22:17:01.000000000 +0200

 +++ glibc-2.5/sysdeps/unix/sysv/linux/alpha/sigsuspend.S	2006-10-22 22:18:05.000000000 +0200

 @@ -31,3 +31,21 @@

  libc_hidden_def (__sigsuspend)

  weak_alias (__sigsuspend, sigsuspend)

  strong_alias (__sigsuspend, __libc_sigsuspend)

 +

 +#ifndef NO_CANCELLATION

 +	.globl __sigsuspend_nocancel;

 +	.align 4;

 +	.type __sigsuspend_nocancel, @function;

 +	.usepv __sigsuspend_nocancel, std;

 +	cfi_startproc;

 +__LABEL(__sigsuspend_nocancel)

 +	ldgp    gp, 0(pv);

 +	PSEUDO_PROF;

 +	PSEUDO_PREPARE_ARGS

 +	lda     v0, SYS_ify(sigsuspend);

 +	call_pal PAL_callsys;

 +	bne     a3, SYSCALL_ERROR_LABEL;

 +	ret;

 +	cfi_endproc;

 +	.size __sigsuspend_nocancel, .-__sigsuspend_nocancel

 +#endif