Changes to !ban and !quiet hostmask processing

Simplified the code paths for the !ban handler. !ban command now
unconditionally passes given parameter through _nick_to_hostmask.

Removed feature where a mask is given yet it still kicks (I don't think
it's ever been used, and it was complicating the code)

Added some simple syntax checking to the _nick_to_hostmask function

Added handlers for invalid syntax to places where _nick_to_hostmask is
called from.

misc edit snuck into this commit: Added a message sent when someone
invokes !m. Just in case it was an accident, they now know how to undo
it.