(server-kill-new-buffers): New user option.

(server-kill-new-buffers): New user option.
(server-existing-buffer): New buffer-local variable.
(server-visit-files): When using an existing buffer, set
server-existing-buffer to t.
(server-buffer-done): If server-kill-new-buffers is t, kill the
buffer, unless it was already present before visiting it with
Emacs server.