hammer --listen/--remote to speed up shell scripting
Cloned from https://bugzilla.redhat.com/show_bug.cgi?id=1122811
Description of problem:
To make shell scripting with Hammer faster, it would be good to have a "--listen" mode which spawns Hammer into the background and then allows "--remote" to connect to the already-running process.
Modelled on guestfish: http://libguestfs.org/guestfish.1.html#remote-control-guestfish-over-a-socket