Add a few accessors for IPAddressFamily and make first use of them

Add a few accessors for IPAddressFamily and make first use of them

One reason why this file is hard to read are endless repetitions of
checks and assignments reaching deep inside structs. This can be made
much more readable by adding a bunch of accessors. As a first step,
we deal with IPAddressFamily, where we want to check the type of the
ipAddressChoice member, check whether the inheritance element is present
or access the addressOrRanges field.

This diff already makes minimal use of these accessors to appease -Werror.
More use and additional accessors will follow in later passes.

ok inoguchi jsing