Fix `server-name' and `server-socket-dir' for (Bug#24218)

Fix `server-name' and `server-socket-dir' for (Bug#24218)

* lisp/server.el: (server-external-socket-initialised): New
(server-name): Compute server name from `get-external-sockname'.
(server-socket-dir): Compute socket dir from
`get-external-sockname'.
(server-start): Don't check for existing server when an
uninitialised external socket has been passed to Emacs.
* src/emacs.c: (main): Obtain socket name via getsockname and pass
to `init_process_emacs'.
* src/lisp.h: (init_process_emacs): Add second parameter.
* src/process.c: (external_sock_name): New.
(get-external-sockname): New.
(init_process_emacs): Set `external_sock_name' to `sockname'
parameter.