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	2008-03-13 00:54:10.000000000 +0100

 +++ duma_2_5_14/Makefile	2008-06-17 10:28:45.000000000 +0200

 @@ -93,10 +93,6 @@

  # also define 'WIN32'

  # some defaults:

 -CC=gcc

 -CXX=g++

 -AR=ar

 -RANLIB=ranlib

  INSTALL=install

  RM=rm

  RMFORCE=rm -f

 @@ -317,7 +313,7 @@

  createconf$(EXEPOSTFIX): createconf.o

  	- $(RMFORCE) createconf$(EXEPOSTFIX)

 -	$(CC) $(CFLAGS) $(DUMA_OPTIONS) createconf.o -o createconf$(EXEPOSTFIX)

 +	$(HOSTCC) $(HOSTCFLAGS) $(DUMA_OPTIONS) createconf.o -o createconf$(EXEPOSTFIX)

  tstheap$(EXEPOSTFIX): libduma.a tstheap.o

  	- $(RMFORCE) tstheap$(EXEPOSTFIX)

 @@ -378,7 +374,7 @@

  # define rules how to build objects for createconf

  #

  createconf.o:

 -	$(CC) $(CFLAGS) $(DUMA_OPTIONS) -c createconf.c -o $@

 +	$(HOSTCC) $(HOSTCFLAGS) $(DUMA_OPTIONS) -c createconf.c -o $@

  #