which can take multiple values -- "pax" or "gtar". The default value
of EXTRACT_USING is "pax", which more closely matches reality since
before, we were using bootstrap "tar" for ${GTAR} and it was actually
pax-as-tar. Also, stop pretending pax-as-tar from the bootstrap kit
or on NetBSD is GNU tar. Lastly, in bsd.pkg.extract.mk, note whether
we need "pax" or "gtar" depending on what we need to extract the
distfiles.
