nrelease: Reduce the size of /usr/local on the IMG/ISO considerably.

nrelease: Reduce the size of /usr/local on the IMG/ISO considerably.

* Exchange git with git-lite which should have everything we need for
  our purposes. This gets rid of perl and various p5-* packages.

* Build the bind-tools without option PYTHON. This gets rid of python
  and various py27-* packages.

* While here, use <category>_<port>_{,UN}SET to handle options, instead
  of rolling our own mechanism.

All in all, this commit cuts the number of installed packages from 40
to 22 and reduces /usr/local's size from 300M to 145M.

Thanks to zrj for help and advice.