Add native dports support
DPorts, the ports building system tailored for Dragonfly, requires
five bsd.*.mk scripts to be placed at /usr/share/mk in order to work
on DragonFly without any type of bootstrapping.
After installing dports using the /usr/Makefile script, any dport can
be built using the same instructions as FreeBSD (e.g. make install clean)
The resulting packages will be handled by "pkg", which will also be used
in the future for downloading prebuild binaries from an official
repository.