Add function in bufferinfo.cpp to report if chat messages are accepted

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.