debchange: ubuntu native packages version increment support

this supports ubuntu native packages with the form ${version}ubuntu

the native package versioning discussion on the ubuntu-devel mailing list seemed to favor ubuntu without a number as suffix for native packages, but dch/debchange wasn't able to properly increment this version number.

this patch adds support, so now versions are incremented like this:

old	new
1ubuntu	2ubuntu
1.2ubuntu	1.3ubuntu
3.1.2ubuntu.build10	3.1.3ubuntu
3.1.2ubuntu	3.1.2ubuntu.build1 for rebuild