Rewrite in_control(), so that it is comprehendable without getting mad.

o Provide separate functions for SIOCAIFADDR and for SIOCDIFADDR, with
  clear code flow from beginning to the end. After that the rest of
  in_control() gets very small and clear.
o Provide sx(9) lock to protect against parallel ioctl() invocations.
o Reimplement logic from r201282, that tried to keep localhost route in
  table when multiple P2P interfaces with same local address are created
  and deleted.

Discussed with:		pluknet, melifaro
Sponsored by:		Netflix
Sponsored by:		Nginx, Inc.
netinet code no longer uses IFA_RTSELF.