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>

 Original patch from gentoo: ../../dev/crosstool-NG/ct-ng.trunk/patches/strace/4.5.17/120-fix-arm-bad-syscall.patch

 -= BEGIN original header =-

 diff -dur strace-4.5.16.orig/syscall.c strace-4.5.16/syscall.c

 -= END original header =-

 diff -durN strace-4.5.17.orig/syscall.c strace-4.5.17/syscall.c

 --- strace-4.5.17.orig/syscall.c	2008-05-28 01:18:29.000000000 +0200

 +++ strace-4.5.17/syscall.c	2008-10-24 21:30:13.000000000 +0200

 @@ -1016,6 +1016,15 @@

  		/*

  		 * Note: we only deal with only 32-bit CPUs here.

  		 */

 +

 +		if (!(tcp->flags & TCB_INSYSCALL) &&

 +		    (tcp->flags & TCB_WAITEXECVE)) {

 +			/* caught a fake syscall from the execve's exit */

 +			tcp->flags &= ~TCB_WAITEXECVE;

 +			return 0;

 +		}

 +

 +

  		if (regs.ARM_cpsr & 0x20) {

  			/*

  			 * Get the Thumb-mode system call number