openzfs: We are FreeBSD, not posix.

Partially revert:
https://github.com/openzfs/zfs/commit/99d7453b43dc0ef04a35e461ef14db72e1326c7c
which introduced this file upstream. This causes this definition to be
preferred for all builds. Make the smallest change possible to this file
to change the default to FreeBSD. We're talking to the upstraem folks
about the right fix. Feel free to revert this in the future, so long as
bectl activate -t still works properly afterwards.

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