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 -durN gcc-4.2.1.orig/libstdc++-v3/include/ext/rope gcc-4.2.1/libstdc++-v3/include/ext/rope

 --- gcc-4.2.1.orig/libstdc++-v3/include/ext/rope	2006-10-17 13:56:21.000000000 +0200

 +++ gcc-4.2.1/libstdc++-v3/include/ext/rope	2007-08-03 20:34:23.000000000 +0200

 @@ -58,6 +58,9 @@

  #include <bits/allocator.h>

  #include <ext/hash_fun.h>

 +/* cope w/ index defined as macro, SuSv3 proposal */

 +#undef index

 +

  # ifdef __GC

  #   define __GC_CONST const

  # else

 diff -durN gcc-4.2.1.orig/libstdc++-v3/include/ext/ropeimpl.h gcc-4.2.1/libstdc++-v3/include/ext/ropeimpl.h

 --- gcc-4.2.1.orig/libstdc++-v3/include/ext/ropeimpl.h	2006-10-17 13:56:21.000000000 +0200

 +++ gcc-4.2.1/libstdc++-v3/include/ext/ropeimpl.h	2007-08-03 20:34:23.000000000 +0200

 @@ -54,6 +54,9 @@

  #include <ext/memory> // For uninitialized_copy_n

  #include <ext/numeric> // For power

 +/* cope w/ index defined as macro, SuSv3 proposal */

 +#undef index

 +

  _GLIBCXX_BEGIN_NAMESPACE(__gnu_cxx)

    using std::size_t;