[cleanup] Want to have native bindings to sys/acl

[smklein]

#7888 aims to make mountpoint directories "safer" against mis-use (e.g., writing to the directory before the dataset-under-interest is mounted).

To do this, we want to:

• Check if immutability is supported (currently via ls -d/v)
• Make the directory mutable/immutable (via chmod S+ci)

It should be possible to accomplish this with native bindings to illumos, but I think this will required bindings to https://illumos.org/man/2/acl (sys/acl.h).