Wrap some of the functions used only in main into an anonymous namespace;
some of them were already marked as "static", others were not (but they
were still not needed from anywhere else).
This makes it immediately clear that those functions are only used in the
context of main.
FindUndoPos had a declaration first and the definition later, but it is
fine to just move the definition to where the declaration was. This
simplifies the code further (and is a pure move in the context of
this commit).

This is a bunch of related and straight-forward cleanups to the code
around masternode configuration: Pass arguments as const& instead of
by value, mark functions as const, and move helper functions from
being static in a class and declared inside the header to just being
inside an anonymous namespace in the cpp file.