sysutils/ups-nut: Merge improvements from wip

  - install python interface to nut.  While in theory it probably
    ought to be in a split package, it seems overall useful relative
    to the burden of python, given that python is very likely
    installed anyway.
  - don't run asciidoc (the package didn't depend on it)
  - clean up MESSAGE_SUBST as MESSAGE is long gone
  - enable power down; it has been commented out

(gdt)