-arch and -indep overrides

-arch and -indep overrides

* dh: Now you can use override_dh_command-arch and override_dh_command-indep
  to run different overrides when building arch and indep packages. This
  allows for a much simplified form of rules file in this situation, where
  build-arch/indep and binary-arch/indep targets do not need to be manually
  specified. See man page for examples.
* dh: Note that if a rules file has say, override_dh_fixperms-arch,
  but no corresponding override_dh_fixperms-indep, then the unoverridden
  dh_fixperms will be run on the indep packages.
* dh: Note that the old override_dh_command takes precidence over the new
  overrides, because mixing the two types of overrides would have been
  too complicated. In particular, it's difficult to ensure an
  old override target will work if it's sometimes constrained to only
  acting on half the packages it would normally run on. This would be
  a source of subtle bugs, so is avoided.