Set OPTIONS_UNSET in the argument list to env(1), and add

Set OPTIONS_UNSET in the argument list to env(1), and add
AVAHI to the list.  This fixes the textproc/docproj build
seemingly following FLAVORS being added.

Specifically, the problem with the dependency chain here is:
- textproc/docproj depends on print/cups, which sets AVAHI=on
  by default;
- net/avahi-app depends on devel/gobject-introspection, which
  requires python3+;
- graphics/netpbm depends on graphics/mesa-libs, which can
  only be built with python2.7;
- textproc/docproj depends on a number of graphics ports for
  font rendering, etc.

MFC after:	3 days
MFC before:	12.0-BETA3
Sponsored by:	The FreeBSD Foundation