debhelper modular buildsystems (try 3).

debhelper modular buildsystems (try 3).

* New feature - when listing buildsystems, list their status too
  (auto/specified).
* Dh_Buildsystem_Basic.pm renamed to Dh_Buildsystem.pm
* Addressed a few issues expressed in the comments, answered a few
  comments.
* Cache DEB_BUILD_GNU_TYPE value. Performance hit is noticable when
  listing build systems.
* is_auto_buildable() renamed to check_auto_buildable() (again).
  Since there is is_buildable() now, I didn't want to use is_ for
  that method.

Signed-off-by: Modestas Vainius <modestas@vainius.eu>