FIX: Post Mortem Stacktrace Creation

This fixes the issue, that stack traces were not created properly after cvmfs has dropped it's user credentials. When our custom signal handler is invoked it now resets all signal handlers to their default. This allows the watchdog process to trigger core dump creation by killing the cvmfs2 process using the same signal that was initially raised.

Additionally to the fix I added two test cases that verify the proper stacktrace creation both in an automount and a private mount setup.