Previously, /znc setbuffer had a hardcoded limit of 500, other ways of setting
the buffer size didn't have any limit built-in.
This patch makes that limit configurable and makes sure the various places
honour it.
Thanks to DarthGandalf for the idea with the bForce argument, I were too
small-minded to come up with it myself. :(
git-svn-id: https://znc.svn.sourceforge.net/svnroot/znc/trunk@2020 726aef4b-f618-498e-8847-2d620e286838