FreeBSD: Wire projects support

[amotin]

While FreeBSD itself does not support projects, I don't know a reason why it can't be controlled via zfs project and other subcommands. Most of the code is actually already there and just needs some revival and sync with Linux, plus enabling some tests not depending on the OS support.

How Has This Been Tested?

Did some basic manual testing on FreeBSD by setting/changing/verifying projects with zfs project, creating files/directories and moving them here and there, also observing accounting with zfs projectspace.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Performance enhancement (non-breaking change which improves efficiency)
• [ ] Code cleanup (non-breaking change which makes code smaller or more readable)
• [ ] Quality assurance (non-breaking change which makes the code more robust against bugs)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• [ ] Documentation (a change to man pages or other documentation)

Checklist:

• [x] My code follows the OpenZFS code style requirements.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the contributing document.
• [ ] I have added tests to cover my changes.
• [ ] I have run the ZFS Test Suite with this change applied.
• [x] All commit messages are properly formatted and contain Signed-off-by.