Add new function gst_poll_fd_ignored() for improved Windows compatibility.

Add new function gst_poll_fd_ignored() for improved Windows compatibility.

Original commit message from CVS:
Patch by: Ole André Vadla Ravnås
<ole dot andre dot ravnas at tandberg dot com>
* docs/gst/gstreamer-sections.txt:
* gst/gstpoll.c: (gst_poll_winsock_error_to_errno),
(gst_poll_update_winsock_event_mask),
(gst_poll_prepare_winsock_active_sets),
(gst_poll_collect_winsock_events), (gst_poll_new), (gst_poll_free),
(gst_poll_add_fd_unlocked), (gst_poll_fd_ctl_write),
(gst_poll_fd_ctl_read_unlocked), (gst_poll_fd_ignored),
(gst_poll_fd_has_error), (gst_poll_fd_can_read_unlocked),
(gst_poll_check_ctrl_commands), (gst_poll_wait):
* gst/gstpoll.h:
* win32/common/libgstreamer.def:
Add new function gst_poll_fd_ignored() for improved Windows
compatibility.
Various minor fixes and cleanups. See #520808.