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)

 ripped from Debian

  sysdeps/unix/sysv/linux/posix_fadvise.c |   13 +++++++++++++

  1 file changed, 13 insertions(+)

 diff -durN glibc-2.12.1.orig/sysdeps/unix/sysv/linux/posix_fadvise.c glibc-2.12.1/sysdeps/unix/sysv/linux/posix_fadvise.c

 --- glibc-2.12.1.orig/sysdeps/unix/sysv/linux/posix_fadvise.c	2003-08-17 02:36:22.000000000 +0200

 +++ glibc-2.12.1/sysdeps/unix/sysv/linux/posix_fadvise.c	2009-11-13 00:50:03.000000000 +0100

 @@ -35,6 +35,19 @@

      return INTERNAL_SYSCALL_ERRNO (ret, err);

    return 0;

  #else

 +# ifdef __NR_fadvise64_64

 +  INTERNAL_SYSCALL_DECL (err);  

 +  int ret = INTERNAL_SYSCALL (fadvise64_64, err, 6, fd,

 +			      __LONG_LONG_PAIR ((long) (offset >> 31),

 +						(long) offset),

 +			      __LONG_LONG_PAIR ((long) (len >> 31),

 +						(long) len),

 +			      advise);

 +  if (INTERNAL_SYSCALL_ERROR_P (ret, err))

 +    return INTERNAL_SYSCALL_ERRNO (ret, err);

 +  return 0;

 +# else

    return ENOSYS;

 +# endif

  #endif

  }