dh_builddeb: Build udebs with xz compression, level 1, extreme strategy.

dh_builddeb: Build udebs with xz compression, level 1, extreme strategy.

* dh_builddeb: Build udebs with xz compression, level 1, extreme strategy.
  This has been chosen to not need any more memory or cpu when uncompressing,
  while yeilding the best compressions for udebs. Thanks, Philipp Kern.
* Depend on a new enough dpkg for above features. Backporters will need
  to revert these changes.