irssi Provides: irssi-abi-XXX for safer plugin packaging (Closes: #811445)

Inspired by https://xorg-team.pages.debian.net/xorg/reference/dependencies.html, Florian Schlichting fsfs@debian.org proposed something like this patch to the irssi packaging. This should make it easier to automatically craft narrower dependencies on the debian packaging for irssi plugins.

Signed-off-by: Daniel Kahn Gillmor dkg@fifthhorseman.net