Provide and use a convenience function in bufferinfo.cpp to determine if the buffer accepts "regular" chat messages.

This works around https://github.com/sandsmark/QuasselDroid/issues/98 on the server side by preventing new buffers from being created if the user accidentally (or maliciously) attempts to send chat messages in status buffers.  Previously, this would have screwed up the status buffer requiring a manual DB query to fix.  This should be fixed in Quasseldroid quite soon, but this patch will just ensure that the core's DB does not get corrupted by older Quasseldroid clients if the users do not update.