* src/process.c: Export default filters and sentinels to Elisp.

* src/process.c: Export default filters and sentinels to Elisp.
(Qinternal_default_process_sentinel, Qinternal_default_process_filter):
New constants.
(pset_filter, pset_sentinel, make_process, Fset_process_filter)
(Fset_process_sentinel, Fformat_network_address):
Default to them instead of nil.
(server_accept_connection): Sentinels can't be nil any more.
(read_and_dispose_of_process_output): New function, extracted from
read_process_output.
(read_process_output): Use it; filters can't be nil.
(Finternal_default_process_filter): New function, extracted from
read_process_output.
(exec_sentinel_unwind): Remove function.
(exec_sentinel): Don't zilch sentinel while running.
(status_notify): Sentinels can't be nil.
(Finternal_default_process_sentinel): New function extracted from
status_notify.
(setup_process_coding_systems): Default filter is not nil any more.
(syms_of_process): Export new Elisp functions and initialize
new constants.
* src/lisp.h (make_lisp_proc): New function.