Finish unification of filesystem plugins; finish fs2 support

This PR finishes the implementing support for the "filesystem2" format
for the rest of the platforms that were never converted: aix, windows,
solaris. It also finishes unfiying all filesystem plugins into the same
plugin rather than split across platform directories.

Here's some other cleanup along hte way:
* Fix the fact that "allow_partial_data" only worked on Linux. Oops.
* Add helper for "allow_partial_data" stuff so that "allow_partial_data"
support was easy to be consistent about
* Always get all ZFS options, no reason not to provide all data
* Change the way zfs options are laid out. The existing way separates
everything into two hashes that need be correlated by the user and
doesn't seem to make much sense.

A few notes:
* Solaris had basically no unit tests so I wrote some tests for both the
old and new zfs formats.
* The windows stuff in particular has effectively no testing, but this
is well outside of my expertise... the changes were small so I feel OK
about them.

Signed-off-by: Phil Dibowitz <[email protected]>