Use built-in network primitives.

Use built-in network primitives.
(server-program, server-previous-string): Remove.
(server-previous-strings): New var.
(server-socket-name): New var.
(server-log): Minor change to the output format.
(server-sentinel): Clean up global state when a client disconnects.
(server-unquote-arg): New fun.
(server-start): Use server-socket-name and make-network-process.
(server-process-filter): Now talks to the clients directly.
Normalize file name after unquoting and decoding.
(server-buffer-done): Just close the connection.
(server-switch-buffer): Handle the case where all windows are
dedicated or minibuffers.