Basic parsing of the RTM_NEWADDR message

Receive a netlink message, and if it's an RTM_NEWADDR message print the
address family, prefix length and interface index.
Introduce FAIL() macro

Unconditional abort(), with a comment on why we're aborting.
Only allow AF_INET or AF_INET6

We're only ever going to deal with IP(v4/v6), so add a check to assert
this.
Mark __ABORT and __FAIL as noreturn

We'll never return from __ABORT or __FAIL (as they call abort()). Mark
them as such so the compiler is aware of this and can potentially
optimize the code path.