stream_dvd: better .ifo probing

stream_dvd.c includes a pseudo-protocol that recognizes .IFO files, and
plays them using libdvdread. This was relatively lazy, and could perhaps
easily trigger with files that just had the .ifo extension.

Make the checks stricter, and even probe the file header. Apparently the
first bytes in an .ifo file are always "DVDVIDEO-VTS", so check for
this.

Refuse to load the main "video_ts.ifo". The plan is to use stream_dvdnav
for it.

This also removes at least 1 memory leak.
stream_dvdnav: allow opening DVD directories directly

Same hack as with stream_dvd.c.

VIDEO_TS.IFO files are now opened via stream_dvdnav.c. Directories
containing a VIDEO_TS.IFO or VIDEO_TS/VIDEO_TS.IFO file are also
opened with it.
stream_bluray: allow opening BDMV directories directly

Similar as the previous commits.

Most of the code is actually copied from the stream_dvdnav.c code, but
I'd rather prefer to duplicate it, than to entangle them. The latter
would probably result in terrible things in a few years.