The availability condition get_description method is called while
gathering data for the modinfo object. As such it is not safe to
call other functions which might rely on modinfo, such as format_string
(if using filters which access modinfo).
This change provides a mechanism to call format_string later, and also
a general callback to do other stuff later as well if needed. It uses
the same approach already taken to make activity names work correctly
in the availability_condition class.
sam marshall
ca7b088d44