Added sys::socket::consts::MSG_*

[maennchen]

Partially solve #316.

[fiveop]

Thank you for the PR. However, #316 is about getting the constants defined in rust-lang/libc and then use those constants for our bitflags. I would prefer adding the constants to libc first, because they have testing infrastructure in place that checks if the constants are defined with the same values in C headers.

[kamalmarhubi]

Ah the description in #316 is a bit unclear. I'll update it.

[homu]

☔ The latest upstream changes (presumably #422) made this pull request unmergeable. Please resolve the merge conflicts.

[kamalmarhubi]

Triaging PRs, and closing this one for inactivity.