Replace macros with const and constexpr

[PoisonNinja]

Macros are bad practices in C++, and everything they do currently can be replaced by const and constexpr.

[PoisonNinja]

This is largely completed. However, some macros may have to be left in for compatibility, mostly the userspace interfacing ones.

This includes all the stat flags/tests, O_, SEEK_, and errno.

[PoisonNinja]

Partially done by #12, but still needs some more work.