Add support for TCP sockets.

Add support for TCP sockets.
(server-use-tcp, server-host, server-auth-dir): New options.
(server-auth-key): New variable.
(server-ensure-safe-dir): Create nonexistent parent dirs.  Ignore Unix-style
file modes on Windows.
(server-start): Crete a TCP or Unix socket according to the value of
`server-use-tcp'.  For TCP sockets, create the id/auth file in `server-auth-dir'
directory.
(server-process-filter): Delete process if authentication fails (which never
happens for Unix sockets).