Add support for channel modes +I and +e.

Add support for the RFC2811 channel modes +I and +e. Their appearance on the wire is more or less equal to +b.

+e specifies exceptions to ban masks (if a user matches both a +b and an +e mode, they are not banned), and +I specifies "automatic invitations" (if a user matches an +I mode on an +i channel, they may enter without being invited).