Use libc specific type for architecture specific ioctl defines on Linux.

[de-vri-es]

This PR should fix the type change of some ioctl constants on Linux introduced by #2530.

It does this by adding a #[doc(hidden)] type called Ioctl, which is defined in libc specific modules and used in arch specific modules.

However, when doing this I noticed that when I added TCGETS2, TCSETS2, ... in #2508, I unconditionally used c_ulong. This is inconsistent with the other ioctl constants for musl and uclibc. This PR also changes those to use the libc specific types. However, PR #2508 has already been released in 0.2.107, so technically that is also a semver incompatible change. The impact is limited to new constants introduced in the last release, and only on musl and uclibc targets.

So what is more important here? Consistency in the type of ioctl constants, or being very strict with backwards compatibility? If it is the latter, I'll revert TCGETS2 etc to c_ulong for this PR.

[rust-highfive]

r? @Amanieu

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[de-vri-es]

Adjusted the definition now to be pub(crate) when cfg(doc) is enabled. This ensures that the documentation says c_ulong /c_int instead of Ioctl with a broken link.

Since cfg(doc) is newer than pub(crate), this shouldn't even break doc generation on older versions of rust.

[JohnTitor]

The latest version was released 12 days ago, we could assume the impact of breaking changes is quite small (, or even nothing if you don't depend on that). I think it's a reasonable trade-off so am going to r+. Thanks for the nice cleanup!

[JohnTitor]

r? @JohnTitor @bors r+

[bors]

📌 Commit 7c95819 has been approved by JohnTitor

[bors]

⌛ Testing commit 7c95819 with merge 8802703...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13
Approved by: JohnTitor
Pushing 8802703 to master...