Fix Debian packaging and make setcap respect DESTDIR
Because we now build a single package, DESTDIR is actually
`debian/<package-name>` rather than `debian/tmp`
(see https://www.debian.org/doc/manuals/debmake-doc/ch05.en.html#rules).
This means that we actually don't need to add `usr` to be installed;
it's already installed to the staging directory.
Additionally, setcap now respects the DESTDIR variable (used when
building the Debian package).