Auto merge of #2097 - cmusser:topic/dfly-waitid, r=JohnTitor
Auto merge of #2097 - cmusser:topic/dfly-waitid, r=JohnTitor

dragonflybsd: expose waitid() prototype + related constants

This exposes the POSIX waitid() process management function and some related defined constants for dragonflybsd. It includes one correction: WSTOPPED which previously had the wrong value. Noticed this when a crate that depended on it, process_control, wouldn't compile on this platform.

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