Fix subproc listening when setting filter to non-t (Bug#36591)

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.