Define enum eContextResult and use its values for returns, instead of just returning 1, 0, or -1 (and always having some comment that explains what -1 means).

This also cleans up the mixup between returning 0 and false, and 1 and true. An inconsistency was discovered during this cleanup, and marked with TODO(sybren). It's not fixed here, as it would consititute a functional change.

The enum isn't used everywhere, as enums in C and C++ can have different storage sizes. To prevent issues, callback functions are still declared as returning`int`. To at least make things easier to understand for humans, I marked those with int /*eContextResult*/.

No functional changes.

This is a followup of D9090, and is intended to unify how context callbacks return values. This will make it easier to extend the approach in D9090 to those functions.