bugfix: monster races that cannot learn skills, will not try to study.
This fixes https://bugs.eressea.de/view.php?id=2054
also prevent monsters from studying unarmed combat if they cannot learn things.
refactor the unit_can_study functionality into a separate function, we are now using it in multiple places.
Merge pull request #79 from badgerman/bug-2054

Bug 2054: monsters trying to STUDY when they cannot.