Different rules of calculation between transfer of asset and usability as fee payment

If user has reserved tokens, he can transfer them, but they can not be used for fee payment asset.
This is bit hard to explain for users, would be nice to have unified behaviour.

Transfer calculate transferable balance as free − max(frozen − reserved, ED)
Fee payment calculate as free − frozen

Start by locating the transfer and fee-payment balance calculations and compare how reserved, frozen, free, and existential-deposit values are handled. Clarify which unified behavior is intended, then add coverage for reserved-token cases and verify that transferability and fee payment follow the same rule.

Written by the indexing model from the issue text.