scripts: add option to start an interactive debug shell

Add an option that, when a command fails:
- starts an interactive shell with the failed command's environment
- attempts re-execution of the failed command, continues, or aborts
at user's whim.

Before starting the debug-shell, the backtrace is printed.
When exiting for an abort, the standard error message is printed.

Based on an idea and a patch from: Johannes Stezenbach <js@sig21.net>
http://sourceware.org/ml/crossgcc/2012-09/msg00144.html

Signed-off-by: Johannes Stezenbach <js@sig21.net>
[yann.morin.1998@free.fr: integrate in the fault handler]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Johannes Stezenbach <js@sig21.net>
Patchwork-Id: 191571
Patchwork-Id: 191668

diff -ru gdb-6.8.orig/bfd/sysdep.h gdb-6.8/bfd/sysdep.h

--- gdb-6.8.orig/bfd/sysdep.h	2007-07-03 16:26:42.000000000 +0200

+++ gdb-6.8/bfd/sysdep.h	2011-08-21 13:33:47.000000000 +0200

@@ -131,7 +131,7 @@

 extern PTR realloc ();

 #endif

-#if !HAVE_DECL_STPCPY

+#if !HAVE_DECL_STPCPY && !defined(stpcpy)

 extern char *stpcpy (char *__dest, const char *__src);

 #endif