Add ov_fopen() (like ov_open, but does the fopen within libvorbisfile)

Add ov_fopen() (like ov_open, but does the fopen within libvorbisfile)
and extend ov_open_callbacks() mechanism as means of encouraging use
over ov_open().

Use of ov_open() now strongly discouraged under Windows;
ov_open_callbacks(fd,vf,NULL,0,OV_CALLBACKS_NOCLOSE) as a direct
replacement or ov_fopen() as a convenient substitute.

svn path=/trunk/vorbis/; revision=13289