fixed invalid chars bug, crash bug, logging

Invalid characters to merge - which I *think* might include accented
chars - are now simply dropped, so the merge  compatibility is low but
better than a merge crash!  Files added now check if they'll overlap the
mixed folder, not mod folder - merge rejected every time.  The last
action of a mod merge is now to write it's name in the installed mods
log.  Minor style cleanup.