tracker-extract: Use default signal handler for SIGALRM and SIGABRT

The ALRM handler was introduced in commit
d9d1881c2548e5d6d55fad1e897f8f058ef28696. The alarm() function seems to
no longer be used, so we can remove it.

The ABRT handler was introduced in commit
3f42a4390d48c6a4840a7bcfce74673ebfd23479. I'm not sure of the reason.
Other Tracker processes use the default handler for ABRT, so let's be
consistent.

https://bugzilla.gnome.org/show_bug.cgi?id=734144
Processes shouldn't install the signal handler until the main loop starts

The signal handler we install calls g_main_loop_quit() when SIGTERM or SIGINT
are raised, but this is only useful if the loop was started. Sending SIGTERM or
SIGINT to a Tracker process while it is starting up fails to stop the process,
and triggers the following a warning:

  GLib-CRITICAL **: g_main_loop_quit: assertion 'loop != NULL' failed

Ideally we'd call initialize_signal_handler from an idle once the main
loop has started, but installing it just before the loop starts should
be a big improvement.

https://bugzilla.gnome.org/show_bug.cgi?id=734144
Merge branch 'sam/signal-handling'