Auto merge of #2864 - ivmarkov:master, r=Amanieu
Auto merge of #2864 - ivmarkov:master, r=Amanieu

Socket constants necessary for ESP-IDF support in socket2

I would like to upstream the support for [ESP-IDF](https://github.com/espressif/esp-idf) in the [socket2 crate](https://github.com/esp-rs-compat/socket2). (Which in turn is necessary for Rust async networking support on top of ESP-IDF.)

To tidy up the patch and remove [this](https://github.com/rust-lang/socket2/compare/master...esp-rs-compat:socket2:master#diff-bbdb7821e30b05ce3afbaa9fbc1526c18d6670d65479baaca3beaf4ea6174891R80) and [this](https://github.com/rust-lang/socket2/compare/master...esp-rs-compat:socket2:master#diff-bbdb7821e30b05ce3afbaa9fbc1526c18d6670d65479baaca3beaf4ea6174891R413) TODOs I need these constants defined in the `libc` crate

rust-lang/libc,rust-lang/libc,rust-lang/libc