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)

 Correctly check for g++ existence.

 Copyright 2007 Yann E. MORIN <yann.morin.1998@anciens.enib.fr>

 Licensed to you as dmalloc-5.5.2 is.

 diff -dur dmalloc-5.5.2.orig/configure dmalloc-5.5.2/configure

 --- dmalloc-5.5.2.orig/configure	2007-05-18 11:40:31.000000000 +0200

 +++ dmalloc-5.5.2/configure	2007-05-18 11:42:02.000000000 +0200

 @@ -2643,7 +2643,7 @@

  # see if we actually have a CXX program

 -if test "$ac_cv_prog_CXX" = "" -o ! -x "$ac_cv_prog_CXX"; then

 +if test "$ac_cv_prog_CXX" = "" -o ! -x `which "$ac_cv_prog_CXX"`; then

      { echo "$as_me:$LINENO: WARNING: could not find C++ compiler $ac_cv_prog_CXX" >&5

  echo "$as_me: WARNING: could not find C++ compiler $ac_cv_prog_CXX" >&2;}

      enable_cxx=no