Fix test for ignore-ownership flag on %p volume

`diskutil` takes a volume (mountpoint) only, not a directory or
location on the volume. Parse `df` to find mountpoint of %p. Maybe
this test has never worked? Our test-harness is not verbose enough to
report the skip.
Check that target volume has permissions enabled early in bootstrap

Bootstrapping fails (weird errors when dpkg calling tar to unpack
.deb) in bootstrap phase 3, which is before fink itself runs its
test-suite. Clone t/10features/volume.t permissions-enabled test into
bootstrap shortly after user chooses %p (before any other actual
building begins).