(Fset_process_sentinel): Add sentinel to childp plist

(Fset_process_sentinel): Add sentinel to childp plist
for network process.
(socket_options): Add `:' prefix to option names.  Add optbit field.
(set_socket_option): Remove no_error arg and special handling of s < 0.
Return 1<<optbit for known option, 0 for unknown.
Do not interpret 0 as false for boolean option (only nil).
Pass failed option and value to report_file_error.
(Fset_network_process_options): Replaced by Fset_network_process_option.
(Fset_network_process_option): New function to set just one option.
(Fmake_network_process): Allow :coding arg to be a cons.
Allow :server arg to be an integer specifying backlog size.
Remove :options arg, and allow options to be specified directly
as :KEY, VALUE pairs.  Parse these options before binding socket.
As before, :reuseaddr t is default for a server process, but this
can now be disabled by specifying :reuseaddr nil.
(Fnetwork_interface_info): Rename from Fget_network_interface_info.
(init_process): Availability of network options is now checked with
simpler syntax (featurep 'make-network-process :OPTION); use loop to
setup features.
(syms_of_process): Fix defsubr's for the replaced functions.