Continue gfilenotify.c implementation of missing parts

Continue gfilenotify.c implementation of missing parts

* lisp/filenotify.el (file-notify-add-watch): Append `flags' to
`gfile-add-watch' call.
(file-notify-rm-watch): Modify `file-notify-descriptors' only
after calling the low level functions.

* src/gfilenotify.c (dir_monitor_callback): Check, whether
event_type is expected.
(Fgfile_add_watch): Allow also `change'and `attribute-change' for FLAGS.
(Fgfile_rm_watch): Fix typo.
(syms_of_gfilenotify): Declare Qchange and Qattribute_change.