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 -dur strace-4.5.15.orig/configure.ac strace-4.5.15/configure.ac

 --- strace-4.5.15.orig/configure.ac	2007-07-14 19:25:25.000000000 +0200

 +++ strace-4.5.15/configure.ac	2007-07-14 19:27:35.000000000 +0200

 @@ -91,14 +91,14 @@

  	arch=hppa

  	AC_DEFINE([HPPA], 1, [Define for the HPPA architecture.])

  	;;

 -sh)

 -	arch=sh

 -	AC_DEFINE([SH], 1, [Define for the SH architecture.])

 -	;;

  sh64)

  	arch=sh64

  	AC_DEFINE([SH64], 1, [Define for the SH64 architecture.])

  	;;

 +sh*)

 +	arch=sh

 +	AC_DEFINE([SH], 1, [Define for the SH architecture.])

 +	;;

  x86?64*)

  	arch=x86_64

  	AC_DEFINE([X86_64], 1, [Define for the AMD x86-64 architecture.])