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-2.12.1/sysdeps/unix/sysv/linux/kernel-features.h	2010-07-27 14:34:39.000000000 +0300

 +++ glibc-2.12.1/sysdeps/unix/sysv/linux/kernel-features.h.new	2011-03-10 18:54:37.686795979 +0200

 @@ -437,7 +437,8 @@

     the code.  On PPC they were introduced in 2.6.17-rc1,

     on SH in 2.6.19-rc1.  */

  #if __LINUX_KERNEL_VERSION >= 0x020611 \

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

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

 +    && (!defined __alpha__)

  # define __ASSUME_ATFCTS	1

  #endif