Tweaks to Debian packaging.

Added .so versioning where needed (libpmt, libmblock, libgromnithread) to allow automated dependency checking.

Reverted (perhaps temporarily) to '3.0.svn' version numbering as the '~' trick wasn't working with Ubuntu

Note: the -dev packages don't seem to be sufficient to compile the gr-howto-write-a-block module, so still debugging....