Minor fixups for external socket launching

Minor fixups for external socket launching

* configure.ac (HAVE_LIBSYSTEMD): Change earliest version to 222.
* doc/emacs/misc.texi (Emacs Server):
* etc/NEWS: Spelling and doc fixes.
* src/emacs.c (main) [HAVE_LIBSYSTEMD]:
Check for sd_is_socket returning positive, not zero.
* src/process.c (external_sock_fd): Instead of initializing here ...
(init_process_emacs): ... initialize it here, so that it does the
right thing after dump/restore.
(connect_network_socket): Simplify socket_to_use test.