Implement last remaining bits to update from RFC 4941 to RFC 8981.

Implement last remaining bits to update from RFC 4941 to RFC 8981.
The desync factor is now tracked per temporary IP and the max value
has been increased to about 9 hours.
While here fix a bug where the minimum acceptable pltime in a router
advertisement should be bigger than PRIV_REGEN_ADVANCE not the desync
factor (which didn't make any sense at all).

From Fernando Gont, thanks!
OK sthen