Add FreeBSD jail support hooks

Add the 'zfs jail/unjail' subcommands along with the relevant 
documentation from FreeBSD.  This feature is not supported on
Linux and still requires the match kernel ioctls which will
be included when the FreeBSD platform code is integrated.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9686

lib/libzfs/os/linux/libzfs_mount_os.c

@@ -361,7 +361,7 @@ do_unmount(const char *mntpt, int flags)
 }
 
 int
-zfs_can_user_mount(void)
+zfs_mount_delegation_check(void)
 {
-	return (geteuid() == 0);
+	return ((geteuid() != 0) ? EACCES : 0);
 }