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)

 2009-05-26  Aurelien Jarno  <aurelien@aurel32.net>

 	* sysdeps/alpha/Makefile (CFLAGS-rtld.c): add $(PIC-ccflag).

  ports/sysdeps/alpha/Makefile |    3 ++-

  1 file changed, 2 insertions(+), 1 deletion(-)

 diff -durN glibc-2.10.1.orig/glibc-ports-2.10.1/sysdeps/alpha/Makefile glibc-2.10.1/glibc-ports-2.10.1/sysdeps/alpha/Makefile

 --- glibc-2.10.1.orig/glibc-ports-2.10.1/sysdeps/alpha/Makefile	2009-05-16 10:36:20.000000000 +0200

 +++ glibc-2.10.1/glibc-ports-2.10.1/sysdeps/alpha/Makefile	2009-11-13 00:51:13.000000000 +0100

 @@ -35,7 +35,8 @@

  ifeq ($(subdir),elf)

  # The ld.so startup code cannot use literals until it self-relocates.

 -CFLAGS-rtld.c = -mbuild-constants

 +# It uses more than 64k for the small data area.

 +CFLAGS-rtld.c = -mbuild-constants $(PIC-ccflag)

  endif

  # Build everything with full IEEE math support, and with dynamic rounding;

 diff -durN glibc-2.10.1.orig/ports/sysdeps/alpha/Makefile glibc-2.10.1/ports/sysdeps/alpha/Makefile