[tei-council] updated; mode= of <alt>

[Syd Bauman]

Version on my website is up to r2379.

I'm wondering if the mode= attribute of <alt> and <altGrp>, which
take only the two values "inclusive" and "exclusive" should instead
be the exclusive= attribute which has values "true" and "false".

This change would make better use of datatypes, and would mean one
fewer attribute with the same name and different semantics.