Auto merge of #664 - ndusart:master, r=alexcrichton
Auto merge of #664 - ndusart:master, r=alexcrichton

add poll constants to FreeBSD and DragonFly

cactorium/libc,rust-lang/libc,rust-lang/libc,cactorium/libc,rust-lang-nursery/libc,cactorium/libc,rust-lang-nursery/libc,rust-lang/libc,mneumann/libc,mneumann/libc,rust-lang-nursery/libc,mneumann/libc