* src/mail.cpp (level_4_check): Do the actual check for direct
trust.
(Mail::get_crypto_details): Handle level 4 subcases with different
details.
* src/parsecontroller.cpp (ParseController::parse): Fetch
ultimately trusted keys if a sig was fully or ultimately valid.
(ParseController::get_ultimate_keys): New Helper function
to cache the ultimately valid keys.
* src/parsecontroller.h: Update accordingly.