Improve infrastructure for Unix socket-based local management.

Improve infrastructure for Unix socket-based local management.

"vlog_socket" was essentially a framework for management of a process
over a simple Unix domain socket interface.  Unfortunately it was a little
too simple:

    * It was not extensible for use by clients other than vlog.

    * It was not reliable, since it was based on datagram sockets.

    * It tried to hide itself using poll_fd_callback(), instead of exposing
      itself through the poll loop as does almost every other entity in
      the build tree.

This commit replaces vlog_socket by unixctl, which fixes these problems:

    * Arbitrary commands may now be registered.

    * Use of stream sockets makes it reliable.

    * The interface is exposed to clients.