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: ../4.3.2/320-alpha-signal_h.patch

 -= BEGIN original header =-

 -= END original header =-

 diff -durN gcc-4.3.3.orig/gcc/config/alpha/linux-unwind.h gcc-4.3.3/gcc/config/alpha/linux-unwind.h

 --- gcc-4.3.3.orig/gcc/config/alpha/linux-unwind.h	2006-10-26 21:31:09.000000000 +0200

 +++ gcc-4.3.3/gcc/config/alpha/linux-unwind.h	2009-01-27 22:25:49.000000000 +0100

 @@ -29,6 +29,9 @@

  /* Do code reading to identify a signal frame, and set the frame

     state data appropriately.  See unwind-dw2.c for the structs.  */

 +/* Don't use this if inhibit_libc is set

 +   The build for this target will fail trying to include missing headers */

 +#ifndef inhibit_libc

  #include <signal.h>

  #include <sys/ucontext.h>

 @@ -80,3 +83,4 @@

    fs->retaddr_column = 64;

    return _URC_NO_REASON;

  }

 +#endif /* inhibit_libc */