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 -ur gcc-4.4.1-orig/libjava/Makefile.in gcc-4.4.1/libjava/Makefile.in

 --- gcc-4.4.1-orig/libjava/Makefile.in	2009-08-04 04:12:27.000000000 -0700

 +++ gcc-4.4.1/libjava/Makefile.in	2009-09-23 22:03:15.750951483 -0700

 @@ -8874,6 +8874,9 @@

  ecjx$(EXEEXT): $(ecjx_OBJECTS) $(ecjx_DEPENDENCIES) 

  	@rm -f ecjx$(EXEEXT)

  	$(ecjx_LINK) $(ecjx_LDFLAGS) $(ecjx_OBJECTS) $(ecjx_LDADD) $(LIBS)

 +ecjx.$(OBJEXT): $(ecjx_SOURCES)

 +	@rm -f ecjx.$(OBJEXT)

 +	$(CC_FOR_BUILD) $(BUILD_CFLAGS) -c -o $@ $<

  gappletviewer$(EXEEXT): $(gappletviewer_OBJECTS) $(gappletviewer_DEPENDENCIES) 

  	@rm -f gappletviewer$(EXEEXT)

  	$(gappletviewer_LINK) $(gappletviewer_LDFLAGS) $(gappletviewer_OBJECTS) $(gappletviewer_LDADD) $(LIBS)