* misc.texi (emacsclient Options): Describe what an empty string

* misc.texi (emacsclient Options): Describe what an empty string
argument does for --alternate-editor.

* emacsclient.1: Describe what an empty string argument does for
--alternate-editor.

* emacsclient.c (print_help_and_exit): Describe what an empty
string argument does for --alternate-editor.
(set_socket): Make it possible to not exit in case of an error.
(start_daemon_and_retry_set_socket): New function.
(main): Use it.  Restore the NULL value for socket_name and
server_file after the set_socket call.