should have been set to NULL when the top of a level was being rotated but
instead it was set to the node which pointed down to the level of the deleted
node.

rotate_left and rotate_right no longer take a parent parameter, since they
don't need it with the existence of parent pointers.

dns_rbt_deletefromlevel now takes a pointer to the pointer of the root of
the level as a parameter so that it doesn't have to rediscover the root that
its caller (dns_rbt_deletenode) already discovered.

dns_rbt_deletefromlevel did some (minor) pointless work with the sibling of the
deleted node before the color fixup loop was entered; it does so no more.

forward function declarations changed to ISC style.