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)

 many ports hit this warning:

 ../sysdeps/unix/sysv/linux/check_native.c:46: warning: implicit declaration of function 'memset'

 ../sysdeps/unix/sysv/linux/check_native.c:46: warning: implicit declaration of function 'memset'

 snipped from suse

 diff -durN glibc-2.12.1.orig/sysdeps/unix/sysv/linux/check_native.c glibc-2.12.1/sysdeps/unix/sysv/linux/check_native.c

 --- glibc-2.12.1.orig/sysdeps/unix/sysv/linux/check_native.c	2007-11-24 04:12:17.000000000 +0100

 +++ glibc-2.12.1/sysdeps/unix/sysv/linux/check_native.c	2009-11-13 00:50:11.000000000 +0100

 @@ -23,6 +23,7 @@

  #include <stddef.h>

  #include <stdint.h>

  #include <stdlib.h>

 +#include <string.h>

  #include <time.h>

  #include <unistd.h>

  #include <net/if.h>