Use dpkg-source built-in quilt support instead of full quilt (Closes: #1053465, #1053466)

unicorn does not need to Build-Depends on quilt. dpkg-source built-in support for quilt patches (already integrated in the standard dh pipeline) is enough.

This MR contains an addition commit that sets up d/clean to remove various files generated during the build process and that must be cleaned up during the clean phase.