add missing socket constants

[ndusart]

Add some missing socket constants that are hardcoded by nix (nix-rust/nix#647)

I took the opportunity to merge some constants in a upper module when applicable.

[ndusart]

Fixes #655

[ndusart]

I've got some problems defining SO_RESTRICTION and SO_RESTRICT_* constants for apple platforms.

These are defined in sys/socket.h: https://github.com/opensource-apple/xnu/blob/dc0628e187c3148723505cf1f1d35bb948d3195b/bsd/sys/socket.h#L188-L190
in a #ifdef PRIVATE block, but even when defining it in build.rs (using cfg.define("PRIVATE", None)), it fails to get these constants.

@alexcrichton Do I misused the ctest generator for this ?

[alexcrichton]

Nah I can see -DPRIVATE passed but it looks like that's not working? Maybe these can't be defined for OSX yet?

[ndusart]

Yes maybe :/

I removed them from this PR then. I will discuss in nix if they keep them.

[Susurrus]

Reproducing my comment in nix-rust/nix#647 here:

Those constants don't actually exist on Mac as of 10.11.6 (I have one locally). I did grep SO_RESTRICT -r /usr/include and turned up nothing. Additionally I looked through /usr/include/sys/socket.h and those constants are gone (though the values aren't reused by other constants).

I think the PRIVATE constant is a good hint that these should never have been used by external code anyways, so it's unsurprising that they've removed them. I don't think they should be added to libc.

[ndusart]

OK it's RTM then.

[ndusart]

Just added missing SOCK_RDM for linux and android.

Really ready to be merged now :)

[alexcrichton]

@bors: r+

[bors]

📌 Commit 9f5766f has been approved by alexcrichton

[bors]

⌛ Testing commit 9f5766f with merge 472ffbb...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 472ffbb to master...