Introduce an enum that contains only AF_INET/AF_INET6. This allows us to remove
the 'default: panic()' case from a number of switch statements, simplifying the
code a litte.
We #ifdef the enum values so that the compiler does not complain if we do not
compile e.g. AF_INET in a NOINET kernel.
Suggested by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")