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 -dur libelf-0.8.10.orig/configure libelf-0.8.10/configure

 --- libelf-0.8.10.orig/configure	2007-09-07 14:08:06.000000000 +0200

 +++ libelf-0.8.10/configure	2008-10-23 22:43:19.000000000 +0200

 @@ -1595,7 +1595,7 @@

    echo $ac_n "(cached) $ac_c" 1>&6

  else

    if test "$cross_compiling" = yes; then

 -  ac_cv_sizeof_long_long=0

 +  ac_cv_sizeof_long_long=8

  else

    cat > conftest.$ac_ext <<EOF

  #line 1602 "configure"

 diff -dur libelf-0.8.10.orig/configure.in libelf-0.8.10/configure.in

 --- libelf-0.8.10.orig/configure.in	2007-09-07 14:07:59.000000000 +0200

 +++ libelf-0.8.10/configure.in	2008-10-23 22:43:10.000000000 +0200

 @@ -90,7 +90,7 @@

  AC_CHECK_SIZEOF(short,2)

  AC_CHECK_SIZEOF(int,4)

  AC_CHECK_SIZEOF(long,4)

 -AC_CHECK_SIZEOF(long long,0)

 +AC_CHECK_SIZEOF(long long,8)

  # Windows port

  AC_CHECK_SIZEOF(__int64, 0)