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

Fix for psim build failure due to -lz

See http://sourceware.org/bugzilla/show_bug.cgi?id=12202

--- gdb-7.2.orig/sim/ppc/Makefile.in

+++ gdb-7.2/sim/ppc/Makefile.in

@@ -551,7 +551,7 @@

 PACKAGE_OBJ = @sim_pk_obj@

-psim: $(TARGETLIB) main.o $(LIBIBERTY_LIB) $(BFD_LIB) $(LIBS) $(LIBINTL_DEP)

+psim: $(TARGETLIB) main.o $(LIBIBERTY_LIB) $(BFD_LIB) $(LIBINTL_DEP)

 	$(CC) $(CFLAGS) $(SIM_CFLAGS) $(LDFLAGS) -o psim$(EXEEXT) main.o $(TARGETLIB) $(BFD_LIB) $(LIBINTL) $(LIBIBERTY_LIB) $(LIBS)

 run: psim