color: make a test for curse availability explicite We won't use terminfo when curse failed to load. Before this change, we were doing an indirect test, relying on the fact some variable ('_terminfo_params') would be empty if curses failed to load. We update the code to be more explicit and directly checks if we managed to load the curse module.

color: move 'win32' declaration to the core module This is another part of moving color implementation into core. before we can move the advance logic, we need to move the basic definition and building bricks. This is one more step on that road.

smartset: preserve istopo for baseset operations This is a follow-up of "smartset: use native set operations as fast paths". It's more correct to just preserve the "istopo" information for "&" and "-" operations, like what filteredset does.