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 duma_2_5_14.orig/Makefile duma_2_5_14/Makefile

 --- duma_2_5_14.orig/Makefile	2008-06-17 10:30:40.000000000 +0200

 +++ duma_2_5_14/Makefile	2008-06-17 10:30:17.000000000 +0200

 @@ -228,9 +228,13 @@

  	dumatest.c tstheap.c thread-test.c testmt.c dumatestpp.cpp testoperators.cpp \

  	createconf.c

 -OBJECTS = dumapp.o duma.o sem_inc.o print.o

 +OBJECTS = duma.o sem_inc.o print.o

 +SO_OBJECTS = duma_so.o sem_inc_so.o print_so.o

 -SO_OBJECTS = dumapp_so.o duma_so.o sem_inc_so.o print_so.o

 +ifeq ($(DUMA_CPP),1)

 +OBJECTS += dumapp.o

 +SO_OBJECTS += dumapp_so.o

 +endif

  all:	libduma.a tstheap$(EXEPOSTFIX) dumatest$(EXEPOSTFIX) thread-test$(EXEPOSTFIX) testmt$(EXEPOSTFIX) dumatestpp$(EXEPOSTFIX) testoperators$(EXEPOSTFIX) $(DUMA_DYN_DEPS)