mythmusic fixes for multiple discs

[ijc]

Ref: ​https://github.com/MythTV/mythtv/pull/191

I noticed that mythmusic wasn't handling multi-disc albums correctly.

• the flacvorbis metadata handler didn't know about the DISCNUMBER tag so I added it (I added TOTALTRACKS and TOTALDISCS while I was there too). My goto reference for tag names is ​https://picard.musicbrainz.org/docs/mappings/ (all my music is tagged using Picard so this is what I've ended up with).
• once the discnumber was in the DB I noticed that the album sort wasn't handling it correctly, e.g. for a two disc album it would sort disc.track-wise as 1.1, 2.1, 1.2, 2.2, etc rather than 1.1, 1.2, ..., 2.1, 2.2. Incorporate the disc number into the album order to correct this. I believe a single CD can have at most 99 tracks so use a multiplier of 100. This means a 10 disc box set would clash with album map order offset of 1000, so bump that to 10000.

I've been running this on my v30 based production system for a week and have tested against master. I don't know if it is too late for v31.0 but might be nice to have for v31.1?