API: Include variant name in meta=siteinfo

For languages with variants, we're already returning the list of variant
codes in meta=siteinfo&siprop=general. Clients may have use for the
variant's human-readable name too, and we can easily add it to the
datastructure.

Change-Id: I6c7ebf43a699a0b2346cb0fdeef6487217992f5c
Merge "API: Include variant name in meta=siteinfo"