Refactoring: move net specific file descriptor methods

[TobiasSchaffner]

Move the implementations of net specific file descriptor methods from
io to net. This makes it easier to exclude net at all if it is not needed
for a target.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @BurntSushi (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit 2ccaeff has been approved by alexcrichton

[bors]

⌛ Testing commit 2ccaeff with merge c13aea02549e8e4f4aaad3ed62d6fa6c2866affc...

[bors]

💔 Test failed - status-travis

[kennytm]

@bors retry

#43283 on arm-android.

[01:09:26] thread '<unnamed>' panicked at 'other was less than the current instant', /checkout/src/libstd/sys/unix/time.rs:276:16
[01:09:31] test sync::mpsc::tests::stress_recv_timeout_two_threads ... ok
[01:09:34] test collections::hash::map::test_map::test_lots_of_insertions ... ok
[01:09:57] test process::tests::test_process_output_fail_to_start has been running for over 60 seconds


No output has been received in the last 30m0s, this potentially indicates a stalled build or something wrong with the build itself.

[bors]

⌛ Testing commit 2ccaeff with merge 85cdc54543978fffd351644a571e59b4ff0fb0c1...

[kennytm]

@bors retry

The macOS 64-bit dist job stuck at the "Job received" stage for over 3 hours and never start.

[kennytm]

↑ https://travis-ci.org/rust-lang/rust/builds/265581807 (not editing to prevent sending a bors command)

Problem on Travis side probably caused by the recent update https://www.traviscistatus.com/incidents/kdvnlc4w3n67.

cc #40672

[bors]

⌛ Testing commit 2ccaeff with merge 8016eea...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 8016eea to master...