Auto merge of #2415 - ghedo:sock_txtime, r=JohnTitor
Auto merge of #2415 - ghedo:sock_txtime, r=JohnTitor

Add struct sock_txtime and related flags

These are needed to use the SO_TXTIME socket option on Linux, which is
already exposed.

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