Later, we should move all such objects. And we should rename
the API to have a unique prefix, like "NMPPlObjIP[4]Address".
This is just a first step that introduces more inconsistencies than it
solves. It will get better afterwards.
"nmp-base.h" really should only contain simple defines like enum types
or #define. As such, it almost does not need a source file.
However, the enum-to-string methods for the enums of "nmp-base.h" need a
place. Add "nmp-base.c" for that.
