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)

--- glibc-2.7/sysdeps/unix/sysv/linux/kernel-features.h	2007-08-14 06:21:09.000000000 +0300

+++ glibc-2.7/sysdeps/unix/sysv/linux/kernel-features.h.new	2008-07-23 07:05:20.000000000 +0300

@@ -454,8 +454,7 @@

    and on Alpha just after 2.6.22-rc1.  */

 #if __LINUX_KERNEL_VERSION >= 0x020611 \

     && ((!defined __sh__ && !defined __alpha__) \

-	|| (__LINUX_KERNEL_VERSION >= 0x020613 && defined __sh__) \

-	|| (__LINUX_KERNEL_VERSION >= 0x020617 && defined __alpha__))

+	|| (__LINUX_KERNEL_VERSION >= 0x020613 && defined __sh__))

 # define __ASSUME_ATFCTS	1

 #endif