Validate prefix information in router advertisements according to RFC

Validate prefix information in router advertisements according to RFC
4862 Section 5.5.3.

This very likely solves the problem of slaacd generating privacy
addresses at a very high rate as reported by Matthias Schmidt on
bugs@; thanks!

The problem is that we constantly generate new privacy addresses if we
receive a router advertisement with a pltime of 0 since that address
will immediately be deprecated.

This needs revisiting since we will run into the same problem with
other low pltimes.