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)

diff -dur strace-4.5.15.orig/signal.c strace-4.5.15/signal.c

--- strace-4.5.15.orig/signal.c	2007-01-11 23:08:38.000000000 +0100

+++ strace-4.5.15/signal.c	2007-07-14 19:24:43.000000000 +0200

@@ -1440,7 +1440,7 @@

 	  	tcp->u_rval = tcp->u_error = 0;

 		if(tcp->u_arg[0] == 0)

 		  	return 0;

-		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1]);

+		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1], 0);

 		return RVAL_NONE | RVAL_STR;

 	}

 	return 0;