Socket MAC labels so_label and so_peerlabel are now protected by

Socket MAC labels so_label and so_peerlabel are now protected by
SOCK_LOCK(so):

- Hold socket lock over calls to MAC entry points reading or
  manipulating socket labels.

- Assert socket lock in MAC entry point implementations.

- When externalizing the socket label, first make a thread-local
  copy while holding the socket lock, then release the socket lock
  to externalize to userspace.