/usr/Makefile: Switch to DragonFlyBSD repo on GitHub

I transferred the ownership of both the DPorts and the DeltaPorts
repositories from the jrmarino account to the DragonFlyBSD account.

All references to the jrmarino account still work, but updating the
makefile to reflect the DragonFlyBSD account looks more official.
/usr/Makefile: Fix obsolete references to Github

Only the tarball comes from Github.  The git repository comes from
git.dragonflybsd.org.  Change some of the descriptions to match what
is in place for the src repository.