Make make-{network,serial}-process handle :coding nil consistently

Make make-{network,serial}-process handle :coding nil consistently

The handling of :coding nil was different between
make-{network,serial}-process and make-{pipe}process.  Now they all
handle :coding nil as if :coding had not been specified.

* process.c (Fmake_serial_process)
(set_network_socket_coding_system): Use plist-get to check if
:coding has been specified instead of plist-member, to ensure that
":coding nil" does not override coding-system-for-{read,write}.

* network-stream-tests.el (check-network-process-coding-system-bind)
(check-network-process-coding-system-no-override)
(check-network-process-coding-system-override): New tests.

* etc/NEWS: Describe change in make-network-process and
make-serial-process :coding behavior.