Warn the user if upgrade contains no kernels

Various problems can arise if the upgrade contains no kernels, since we
don't build a new initramfs. (Normally we should get a new kernel with
the upgrade but sometimes there's upgrade path problems.)

So: check the upgrade transaction for any package that matches the list
of kernel package names, and warn the user if there's nothing there.

(Yes, in theory we could look through the rpmdb and force-install a
kernel package from the new distribution, but this violates the "no
special cases for packages" principle.)
Treat --network VERSION argument as a float

There are some Fedora-derived distros that use floats for version
numbers (e.g. RHEL 6.5), so we should allow versions to be floats.

As far as I can tell, the last non-float version number in a Red Hat
release was the RHL 9.0.x beta, in 2003.