Fix definition of crypto_box_curve25519xsalsa20poly1305_macbytes

This function definition was created by an automated script that
incorrectly handled a corner case.
Add remaining functions for looking up constants

The automated script that generated functions for looking up #define'd
constants didn't handle edge cases in these files, so these have been
added by hand. They're thus either more likely or less likely to
contain mistakes (depending on one's particular point of view).