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://www.nabble.com/arm-linux-compilation-failure-and-possible-fix-td19229074.html

 diff -durN glibc-2.9.orig/ports/sysdeps/unix/sysv/linux/arm/sysdep.h glibc-2.9/ports/sysdeps/unix/sysv/linux/arm/sysdep.h

 --- glibc-2.9.orig/ports/sysdeps/unix/sysv/linux/arm/sysdep.h 25 Nov 2008 16:37:26 -0000 1.33

 +++ glibc-2.9/ports/sysdeps/unix/sysv/linux/arm/sysdep.h 27 Jan 2009 15:35:38 -0000

 @@ -28,6 +28,8 @@

  /* Defines RTLD_PRIVATE_ERRNO and USE_DL_SYSINFO.  */

  #include <dl-sysdep.h>

 +#include <tls.h>

 +

  /* For Linux we can use the system call table in the header file

 	/usr/include/asm/unistd.h

     of the kernel.  But these symbols do not follow the SYS_* syntax