config: add and use index-of helpers for cat/subcat array iteration

these 'index-of' helpers find the index for the given cat/subcat. this
allows us to de-duplicate the code from the other helper functions (both
for those that already exist and ones to be added in further commits).
they loop based upon array length rather than looking out for some
particular condition of varying properties in an end marker record,
allowing us to also drop those.

i used an index-of design here rather than have functions that return
pointers because there will be a secondary use for index-of helpers in
later commits.