Rename UnixSocket to UnixDatagram
Rename UnixSocket to UnixDatagram

UnixSocket is too easy to confuse with UnixStream. UnixDatagram is a
kind of weird name since "datagram" is more closely associated with an
individual message than the socket itself, but it's at least
descriptive.

tailhook/rust-unix-socket,rust-lang-nursery/unix-socket,teisenbe/rust-unix-socket,sfackler/rust-unix-socket