icmp6-util: Bind Router Advertisement socket

icmp6-util: Bind Router Advertisement socket

Reuse and refactor the functionality already present for Router
Solicitations in order to create a socket for sending Router
Advertisements. Anticipate reception of incoming Router
Solicitations by setting the ICMPv6 filter accordingly. Also set
the unicast hop limit to 255 for ICMPv6 sockets as unicast Router
Advertisments are to be sent in response to Router Solicitations.

Update the Router Solicitation test case code with a function
definition in order to keep the test case working.