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>

 YEM-20070519:

 Patch from Bernhard Fischer <rep.dot.nop@gmail.com> on the uClibc mailing

 list ( http://www.uclibc.org/lists/uclibc/2008-January/018940.html ) above

 the 400-bits_sysnum_h.patch.

 Index: uClibc/Makefile.in

 ===================================================================

 --- uClibc/Makefile.in	(revision 18651)

 +++ uClibc/Makefile.in	(working copy)

 @@ -114,7 +116,7 @@ install: install_runtime install_dev

  RUNTIME_PREFIX_LIB_FROM_DEVEL_PREFIX_LIB=$(shell $(top_srcdir)extra/scripts/relative_path.sh $(DEVEL_PREFIX)lib $(RUNTIME_PREFIX)lib)

  # Installs header files.

 -install_headers:

 +install_headers: headers

  	$(INSTALL) -d $(PREFIX)$(DEVEL_PREFIX)include

  	printf ".svn\n.cvsignore\nCVS\n" > tar_exclude ; \

  	$(TAR) -chf - -X tar_exclude include \