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 ltrace-0.4.orig/Makefile.in ltrace-0.4/Makefile.in

 --- ltrace-0.4.orig/Makefile.in	2008-10-26 15:34:21.000000000 +0100

 +++ ltrace-0.4/Makefile.in	2008-10-26 15:37:40.000000000 +0100

 @@ -5,6 +5,7 @@

  OS		:= @HOST_OS@

  ARCH		:= $(shell echo "@HOST_ARCH@" |sed -e s/i.86/i386/      \

  						   -e s/sun4u/sparc64/  \

 +						   -e s/alpha.*/alpha/  \

  						   -e s/arm.*/arm/      \

  						   -e s/sa110/arm/      \

  						   -e s/ppc64/ppc/      \