[Libguestfs] [REPOST] guestfsd core capture, and virt-rescue clean shutdown

This is a repost of a previously posted set. It's been updated following review comments. [PATCH 1/4] Add a core_pattern debug command Added missing return statements. [PATCH 2/4] Call sync after guestfsd exits Remove redundant sleep 1; sync [PATCH 3/4] Shut down the appliance cleanly [PATCH 4/4] Ignore launch() error in virt-rescue These were previously 2 patches. They are otherwise unchanged.