Migrate Vxids to 64 bit.

[bsdphk]

We need to make some decisions first:

Old tools on new files will obviously not work, but we should make them break nicely. (Specially crafted header to make them reliably barf ?)

New tools should eat old files too.

Reserve the entire top byte (= 6 extra bits) for future use, for instance binary formats.

Have 64 bit vxids as feature for a release-cycle (or for a major release ?)