mount_image.c: More message queue changes

Yet another rewrite of the message sending/receiving functions.  Everything is
now sent through a generic message loop function with callbacks.  At the same
time, another message has been introduced that is sent from the daemon to the
unmount process to provide its pid.  Then, the unmount process can detect if the
filesystem daemon has crashed by polling this pid every second.  (This should
solve the 10 minute timeout problem.)