Add url_get_file_handle(), which is used to get the file descriptor

Add url_get_file_handle(), which is used to get the file descriptor
associated with the I/O handle (e.g. the fd returned by open()). See
"[RFC] rtsp.c EOF support" thread.

There were previously some URI-specific implementations of the same idea,
e.g. rtp_get_file_handles() and udp_get_file_handle(). All of these are
deprecated by this patch and will be removed at the next major API bump.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@17779 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b