[Libguestfs] Generate coredumps of the guestfs appliance

[Matthew Booth]

[PATCH 1/3] Add a core_pattern debug command
The first patch is just a rework of Rich's earlier core dump patch. It turns it
into a debug subcommand so it can be called at any time. This also has the
advantage of explicitly labelling an extremely untidy API as debug.

[PATCH 2/3] Call sync after guestfsd exits
The second patch seems to be required for cores to be dumped reliably. I was
able to generate cores without this patch, but not in the real scenario I was
actually interested in. I suspect that there was a race for the core to be
committed to disk before the appliance exited. With a larger core file and a
busier cache, perhaps it just wasn't winning.

[PATCH 3/3] Shut down the appliance cleanly
The third patch is opportunistic while I was looking at init. I'm pretty sure it
also fixes RHBZ#618556, although I haven't checked explicitly.

Matt