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 -urN glibc-2.12.1-orig/posix/sys/types.h glibc-2.12.1/posix/sys/types.h

 --- glibc-2.12.1-orig/posix/sys/types.h	2010-12-13 11:47:26.000000000 +0100

 +++ glibc-2.12.1/posix/sys/types.h	2011-03-29 14:16:00.374064708 +0200

 @@ -114,7 +114,10 @@

  #ifdef	__USE_BSD

  # ifndef __daddr_t_defined

  typedef __daddr_t daddr_t;

 +#  if ! defined(caddr_t) && ! defined(__caddr_t_defined)

  typedef __caddr_t caddr_t;

 +#   define __caddr_t_defined

 +#  endif

  #  define __daddr_t_defined

  # endif

  #endif

 diff -urN glibc-2.12.1-orig/sunrpc/rpc/types.h glibc-2.12.1/sunrpc/rpc/types.h

 --- glibc-2.12.1-orig/sunrpc/rpc/types.h	2010-12-13 11:47:26.000000000 +0100

 +++ glibc-2.12.1/sunrpc/rpc/types.h	2011-03-29 14:16:32.988910439 +0200

 @@ -80,7 +80,10 @@

  #endif

  #ifndef __daddr_t_defined

  typedef __daddr_t daddr_t;

 +# if ! defined(caddr_t) && ! defined(__caddr_t_defined)

  typedef __caddr_t caddr_t;

 +#  define __caddr_t_defined

 +# endif

  # define __daddr_t_defined

  #endif