- ifnet.if_output() should be called without ifnet.if_serializer being

- ifnet.if_output() should be called without ifnet.if_serializer being
  held.  Add assertion about it in ether_output().
- ether_output_frame() should be called without the ifnet.if_serializer
  being held.  Add assertion in it.
- arp_ifinit() will be called with ifnet.if_serializer being held.  To
  prevent serializer from recursion, ifnet.if_serializer is released
  before calling arprequest(), which supposes caller does not hold output
  iface's serializer.
- ifnet.if_serializer can't be held when calling arp_ifinit2().

Reported-by: dillon@