All function pointer types should be `Option<unsafe extern "C" fn(...) -> ...>`

[tgross35]

#2198 falls into this category, and #4341 fixes at least a subset. We should enforce this rule in the style.rs checks.

Three things here:

1. Option because API may allow a null pointer
2. unsafe because functions may have side effects or be invalid pointers (How should we handle raw function pointers? #4998 for a proposal to adjust the second part)
3. extern "C" because that's what the C side expects