GDB: deal with fatal signals and generate core dumps

This adds two related features:

1) dealing with fatal signals such as SIGSEGV
2) generating core dumps automatically (disabled by default) if requested

So now the behaviour on fatal signals (SISEGV and SIGABRT) for now cause
the test to be skipped. Other ways to deal with this proved to be complex
and not really effective.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #218 from clebergnu/gdb_fatal_signals_v2

GDB: deal with fatal signals and generate core dumps (v2)