erroneous const-ness for some functions

[ghost]

While migrating my codebase toward glm, I identified 3 functions that have wrong const-ness in daemon's public API:

• trap_S_StartSound
• CM_BoxTrace
• trap_R_LightForPoint

This is trivial to fix (add the const keyword for un-modified arguments) or work around (use const_cast), of course, but I'm pretty sure there are other ones, so would probably be a good idea to take some time to hunt for more and fix all of those in a single commit. Changing this will break nothing in API or ABI and can benefit games using daemon a lot, as it allows to improve const-ness, making code easier to maintain.