[avatars] section in modopts.ini now used for 2 things:

default import/export sub-board (default to 'syncdata')
export_freq (default: 7 days)

The user avatars export frequency is now controlled and
user avatars should not be exported unless they've been
recently created or updated or the configured frequency
has elapsed.

There's currently no export frequency control for
collections (ever time you export a .bin file, it'll be exported to
the msg base)