nmap: pin to Python 2, make Python optional

Python 3 is not supported, so use python2Packages.

Python is only required for the `ndiff` and `zenmap` binaries, not the
main `nmap` binary, so disable Python by default to reduce the default
closure size, and add a `withPython` arg to re-enable it. Note that
Zenmap is the graphical program, so passing `graphicalSupport = true`
will also automatically enable Python support.