Fix subproc listening when setting filter to non-t (Bug#36591)
* src/process.c (Fset_process_filter): Call add_process_read_fd according to the state of process filter before it's updated. This restores the correct functioning as it was before 2016-02-16 "Allow setting the filter masks later". Inline the set_process_filter_masks call instead of fixing it that function, because it is also called from connect_network_socket, and we don't want to change the behavior of that function so close to release. * test/src/process-tests.el (set-process-filter-t): New test.