deb configure: rtai build-depends on rtai-modules

The rtai-modules deb contains rtai-config and the headers.  Might should
be split out to an rtai-dev.deb package.
deb control: relax the Conflicts

The flavor packages should conflict with pre-ubc packages, but there's
no convenient way to specify that currently.

When we merge ubc into master we'll tag that point as 2.6.0-pre1,
and make the post-ubc debs conflict with linuxcnc < 2.6.0-pre1.