ipv6: updates to privacy addresses per RFC 4941

ipv6: updates to privacy addresses per RFC 4941

Update the code to handle some of the differences between
RFC 3041 and RFC 4941, which obsoletes it. Also a couple
of janitorial fixes.

- Allow router advertisements to increase the lifetime of
  temporary addresses. This was not allowed by RFC 3041,
  but is specified by RFC 4941. It is useful when RA
  lifetimes are lower than TEMP_{VALID,PREFERRED}_LIFETIME:
  in this case, the previous code would delete or deprecate
  addresses prematurely.

- Change the default of MAX_RETRY to 3 per RFC 4941.

- Add a comment to clarify that the preferred and valid
  lifetimes in inet6_ifaddr are relative to the timestamp.

- Shorten lines to 80 characters in a couple of places.

Change-Id: I4da097664d4b1de7c1cebf410895319601c7f1cc
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: JP Abgrall <jpa@google.com>