make use_spacer an enum

make use_spacer an enum

It was a static int before, holding the (already defined) enum's value,
which doesn't make sense. Also it's used only inside conky.c. Since
enums can't be assigned other values than what are defined, we can skip
checking for illegal spacer value at each invocation of spaced_print,
and then also drop the function name argument, which apparently didn't
make sense at all, because use_spacer is a global option.