applied changes from 64a3b3dde6e25c7b9fa7d1cfc51f364825d7ae6f
through 2b0f393ea00c997bf3ebf4377074aac97d58b8f1

Original commit message:
So, it seems gtkconv checks on stuff in the prpl struct. The lack of
add_buddy in AIM and MSN disabled the Conversation->Add menu item.
Found by deryni.