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)

 Patch from upstream:

 http://strace.cvs.sourceforge.net/viewvc/strace/strace/linux/arm/syscallent.h?r1=1.17&r2=1.18&sortby=file

 2008-11-13  Kirill A. Shutemov  <kirill@shutemov.name>

     * linux/arm/syscallent.h: Fix build on ARM EABI which does not

     provide syscalls socketcall and ipc.

 --- strace-4.5.18.orig/linux/arm/syscallent.h	2008/12/29 20:04:15	1.17

 +++ strace-4.5.18/linux/arm/syscallent.h	2009/01/01 23:20:38	1.18

 @@ -431,6 +431,7 @@

  	{ 5,	0,	printargs,		"SYS_398"	}, /* 398 */

  	{ 5,	0,	printargs,		"SYS_399"	}, /* 399 */

 +#ifndef __ARM_EABI__

  #if SYS_socket_subcall != 400

   #error fix me

  #endif

 @@ -481,3 +482,4 @@

  	{ 4,	TI,	sys_shmdt,		"shmdt"		}, /* 440 */

  	{ 4,	TI,	sys_shmget,		"shmget"	}, /* 441 */

  	{ 4,	TI,	sys_shmctl,		"shmctl"	}, /* 442 */

 +#endif