plugins/elements/: Port to GstPoll. See #505417.

plugins/elements/: Port to GstPoll. See #505417.

Original commit message from CVS:
Patch by: Peter Kjellerstedt <pkj at axis dot com>
* plugins/elements/gstfdsink.c: (gst_fd_sink_render),
(gst_fd_sink_start), (gst_fd_sink_stop), (gst_fd_sink_unlock),
(gst_fd_sink_unlock_stop), (gst_fd_sink_update_fd):
* plugins/elements/gstfdsink.h:
* plugins/elements/gstfdsrc.c: (gst_fd_src_update_fd),
(gst_fd_src_start), (gst_fd_src_stop), (gst_fd_src_unlock),
(gst_fd_src_unlock_stop), (gst_fd_src_create),
(gst_fd_src_uri_set_uri):
* plugins/elements/gstfdsrc.h:
Port to GstPoll. See #505417.