Fix leaking file descriptors in hash_file_raw
Implement CMD_ADD_HASH
Send the full path in messages to the hash daemon

Don't want to depend on hash daemon and client having the same working
directory.
Send CMD_ADD_HASH from the client after hashing
Switch to Unix datagram sockets

This is not entirely successful - the retrying and timeout stuff is
despicable, but allows getting *some* parallelism out of it.