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

[Matthew Booth]

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.