impossible to use <libzfs.h> out-of-tree

hello,

i'm writing an application which wants to interact with ZFS filesystems,
so i thought i would use libzfs via <libzfs.h>.  however, <libzfs.h>
includes <libshare.h>, which exists in src but is not installed.

this means that nothing can use <libzfs.h>, so it should not be
installed; this is a bug, we should not install <libzfs.h> if it
cannot be used.

but does this mean we have no way for out-of-tree utilities to interact
with ZFS except by calling "/sbin/zfs" and parsing the output?

tested on 15.0-CURRENT, commit ~a5fe142e084.