Router Advertisements are sent uniformly distributed between a
minimum and maximum time according to RFC 4861, Section 6.2.4.
Default values from RFC 4861 are for now used as minimum and
maximum Router Advertisement timeouts.
When stopping, a Router Advertisement with a router lifetime set
to zero is sent in order to inform any nodes that the interface
on this host no longer is a router.
Define Router Advertisement prefix structure. Add the Prefix
Information ICMPv6 option defined in RFC 4861 to the prefix
information structure, as it will simplify sending a Prefix
Information option later on. In order to handle endianness
correctly, the structure is redefined here instead of using
the one in netinet/icmp6.h.
Add functions to create and modify prefix information and set
default values as defined in RFC 4861, Section 6.2.1.
