config.h should be included from every .c file, and it should be
included before any other include. Fix that.
(As a side effect of how I did this, this also changes us to
consistently use "config.h" rather than <config.h>. To the extent that
it matters [which is not much], quotes are more correct anyway, since
we're talking about a file in our own build tree, not a system
include.)
Create NMIPConfig as the parent of NMIP4Config and NMIP6Config, and
remove the two subclasses from the public API; while it's convenient
to still have both internally, they are now identical to the outside
world.