Extend pkg-create-dbgsym to support Mono packages

Currently, whether or not debug symbols - .mdb files - are included with a package is a largely ad-hoc decision (e.g. banshee comes with symbols in the main package, Tomboy comes with none). dh_clistrip behaves in a manner very similar to dh_strip by splitting out .mdb files into a distinct package, and Ubuntu's pkg-create-dbgsym service should be extended to make use of it. This should help to save space, AND ensure availability of debugger symbols for all Mono packages.