Handle unsupported file system versions for legacy mountpoints

This commit extends https://github.com/openzfsonosx/zfs/commit/287be44f536e27d377518badd2bd8e2758db2be6

Although the ENOTSUP handling in libzfs_mount.c's zfs_mount() will
kick in for file systems with non-legacy mountpoints, it will not
for legacy mountpoints. Since we do not currently use a separate
mount utility on OS X, manual_mount() is a good place to handle
the issue.