This renames a few functions -- skip_spaces_const,
skip_to_space_const, get_number_const, extract_arg_const -- to drop
the "_const" suffix and instead rely on overloading.
This makes future const fixes simpler by reducing the number of lines
that must be changed. I think it is also not any less clear, as all
these functions have the same interface as their non-const versions by
design. Furthermore there's an example of using an overload in-tree
already, namely check_for_argument.
This patch was largely created using some perl one-liners; then a few
fixes were applied by hand.