Remove "placeholder constants"

[workingjubilee]

It's common for C APIs to define constant values to denote the "last" value in a set of constants, like ELAST or IPPROTO_MAX. This seems prone to weird bugs, and these constants provide very marginal benefit to Rust programmers. Since we're considering some breakage anyways, we should just remove these values where we are made aware of them.

See:

• IPPROTO_MAX has changed on newer versions of Linux #1896
• NetBSD: add two more errno values #3040

[JohnTitor]

👍, before 0.3, I'd like to deprecate these values.

[JohnTitor]

Noticed that we have a lot of *MAX consts and deprecating manually is quite hard. We could write a script but it can cause a false positive, hmm. For now, I've edited the PR template to mention this concern.