I installed cdrtools-ossdvd to be able to write on a dvd writer.   I
then built mklivecd, and ran into two problems:

  It required cdrtools, but I cdrtools-ossdvd would do as well, since
  it's just slightly newer version patched to also handle DVD writers.

  cdrtools is not marked to conflict with cdrtools-ossdvd, even
  though they have not just overlapping but _identical_ PLISTS.

The second is obvious how to fix, but I didn't see how to make a
package depend on one of two alternates.  Do we need a mkisofs.mk in
mk?