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>

--- glibc-2.6.1/sysdeps/unix/sysv/linux/kernel-features.h	2006-07-29 07:43:27.000000000 +0300

+++ glibc-2.6.1/sysdeps/unix/sysv/linux/kernel-features.h.new	2008-07-26 08:45:06.000000000 +0300

@@ -445,7 +445,7 @@

 /* The *at syscalls were introduced just after 2.6.16-rc1.  Due to the way the

    kernel versions are advertised we can only rely on 2.6.17 to have

    the code.  */

-#if __LINUX_KERNEL_VERSION >= 0x020611

+#if __LINUX_KERNEL_VERSION >= 0x020611 && !defined __alpha__

 # define __ASSUME_ATFCTS	1

 #endif