Import Debian helpers

  * Imported from git://git.debian.org/git/pkg-opensc/opensc.git (2e9effe)
Fix Debian helpers to correctly build opensc-opendnie

  * Tested with git-buildpackage and git-pbuilder
Add debian/gbp.conf to make git-buildpackage behave by default as:

  $ git-buildpackage --git-no-pristine-tar --git-upstream-tag=upstream