Allow a default socket value for domain-scoket.

  If no path is specified ~/.lumail.sock is used.
  Allow socket to be default.
  Fixed braces.

  Now "./sytle/style-braces.t" passes.
  Avoid including sys/socket.h more than once.
  build `lumailctl` by default too.
  Install lumailctl too.
  Ignore the generated lumailctl binary