Add wrapper stub for zfs_cmd ioctl to libzpool

FreeBSD needs a wrapper for handling zfs_cmd ioctls.
In libzfs this is handled by zfs_ioctl. However, here
we need to wrap the call directly.

Reviewed-by: Allan Jude <allanjude@freebsd.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9511

include/libzutil.h

@@ -143,6 +143,9 @@ extern void zpool_dump_ddt(const ddt_stat_t *, const ddt_histogram_t *);
 extern int zpool_history_unpack(char *, uint64_t, uint64_t *, nvlist_t ***,
     uint_t *);
 
+struct zfs_cmd;
+int zfs_ioctl_fd(int fd, unsigned long request, struct zfs_cmd *zc);
+
 #ifdef	__cplusplus
 }
 #endif