fix for maildir synced tags in new.tags

[David Bremner]

As I mentioned in [1], it's currently not possible to have maildir
synced tags like 'flagged' in new.tags, which blocks some ideas about
synching tags via maildir (in addition to just being a bug).

This series fixes that. As I write this I remember that it doesn't
bump the minor version of the SONAME, but that can always happen
later.

[1] id:20170804213845.18902-1-david at tethera.net