Create and remove the ICMPv6 Router Advertisement socket file
descriptor and implement Router Advertisment sending. As not
all options are mandatory, use IO vectors to point to the included
options and the prefix information.
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.
