Multiple exports(5) lines generated by ZFS's sharenfs property (patch for review)

Hi,

I just put a patch on phabricator (D45814) that adds support for multiple
sets of options for different hosts/networks to the ZFS sharenfs property.
(It is based loosely on a patch in bugzilla PR#147881, now 14 years old.)

I have listed a couple of reviewers, but please feel free to review it as well.
(It will eventually need to become a pull request for OpenZFS, but I'd like
to run it past FreeBSD'ers first.)

The sets are separated by ';'s. One question is whether ';'s separated by
only whitespace should be allowed. If allowed they generate a "default"
exports line that exports the file system to the world with default options,
such as read/write.
The patch currently on phabricator allows this, but I am thinking that it
should not do so.
--> The default case could still be specified as it is now, as a single set
     of options without any ';'s in the sharenfs property.

Thanks in advance for any reviews, rick