Minor cleanups for server-name fix (Bug#24218)

Minor cleanups for server-name fix (Bug#24218)

* lisp/server.el (server--external-socket-initialized): Rename
from server-external-socket-initialised, since it should be
private and Emacs uses American spelling.  All uses changed.
* src/emacs.c, src/lisp.h: Revert previous changes, as the
initialization is now done in src/process.c, which already
includes the relevant files.
* src/process.c (union u_sockaddr): Move decl to top level.
(external_sock_name, Fget_external_sockname): Remove, replacing
with Vinternal__external_sockname.  All uses changed.
(init_process_emacs): Deduce socket name ourselves rather than
have main.c do it.  Use conv_sockaddr_to_lisp instead of doing
it by hand.  Conditionalize it on HAVE_GETSOCKNAME.