stream_bluray: fix for zero-based title index for Blu-ray

The title for stream_bluray DID start from 1 and I misunderstood that it started from 0 because mpv accepted bd://0 as a proper argument.
In fact, 0 title was an alias for the longest title but it was not handled as a special value. This commit fixes these behavious.

'disc-title' property for Blu-ray now starts from 0 and the default title can be specified by 'longest' title just like stream_dvdnav: bd://longest. Of course, 'longest' can be omitted.

Sorry for messy pull requests.