stream_bluray: use more proper error code for stream control

Use STREAM_OK instead of hardcoded value 1.
Handle failure of setting title as an unsupported behaviour rather
than an error.
stream_bluray: select initial angle only if peeking title succeeded

The angles should be set and queried only if a valid title is
selected. Also, in navigation mode, there are some limitations
which make it impossible to query current title/angle.
stream_bluray: cosmetic refactoring

Remove unused variables. Declare variables when they are needed.
Adjust brackets for mpv's convention. Clean up too many empty lines.
player: rename dvdnav to discnav

Now, navigation works both of DVD and non-BD-J Blu-ray. Therefore,
rename all 'dvdnav' strings which are not DVD specific to 'discnav'