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 glibc-cvs-2.9.orig/debug/readlink_chk.c glibc-cvs-2.9/debug/readlink_chk.c

 --- glibc-cvs-2.9.orig/debug/readlink_chk.c	2005-03-01 01:41:15.000000000 +0100

 +++ glibc-cvs-2.9/debug/readlink_chk.c	2009-06-01 10:59:37.000000000 +0200

 @@ -25,7 +25,7 @@

  ssize_t

 -__readlink_chk (const char *path, void *buf, size_t len, size_t buflen)

 +__readlink_chk (const char *path, char *buf, size_t len, size_t buflen)

  {

    if (len > buflen)

      __chk_fail ();

 diff -dur glibc-cvs-2.9.orig/debug/readlinkat_chk.c glibc-cvs-2.9/debug/readlinkat_chk.c

 --- glibc-cvs-2.9.orig/debug/readlinkat_chk.c	2006-04-24 18:56:12.000000000 +0200

 +++ glibc-cvs-2.9/debug/readlinkat_chk.c	2009-06-01 11:07:26.000000000 +0200

 @@ -21,7 +21,7 @@

  ssize_t

 -__readlinkat_chk (int fd, const char *path, void *buf, size_t len,

 +__readlinkat_chk (int fd, const char *path, char *buf, size_t len,

  		  size_t buflen)

  {

    if (len > buflen)