Add gdb python plugin which analyzes vulnerability

This is an RFC, please do not commit.

Usage:
Run abrt-action-analyze-vulnerability in a directory which contains
./coredump file.
If crash looks exploitable, the tool creates ./exploitable file.
Example such a file:

"""
Likely crash reason: Write to an invalid address
Exploitable rating (0-9 scale): 6
"""

This patch adds abrt-action-analyze-vulnerability invocation to
"EVENT=post-create analyzer=CCpp".

TODO:
* instruction analyzer is x86 specific now, make it per-arch
  (how to get arch name???)

v3: improved error detection (e.g. what to do if gdb failed to run?),
suppressed stray gdb output, set $SIGNO_OF_THE_COREDUMP to work around
non-working $_siginfo.si_signo

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>