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)

 Dmitry V. Levin [Tue, 3 Nov 2009 16:49:49 +0000]

 Fix check for linux/netlink.h on Linux 2.6.32-rc5+

 * configure.ac (AC_CHECK_HEADERS): In check for linux/netlink.h, include 

   sys/socket.h instead of linux/socket.h beforehand.

 diff --git a/configure.ac b/configure.ac

 --- a/configure.ac

 +++ b/configure.ac

 @@ -258,7 +258,7 @@ AC_CHECK_HEADERS([ \

  ], [], [])

  AC_CHECK_HEADERS([linux/icmp.h linux/in6.h linux/netlink.h linux/if_packet.h],

                   [], [], [#include <stddef.h>

 -#include <linux/socket.h>])

 +#include <sys/socket.h>])

  AC_CHECK_HEADERS([asm/sigcontext.h], [], [], [#include <signal.h>])

  AC_CHECK_TYPES([struct sigcontext_struct,

  		struct sigcontext],,, [#include <signal.h>])