Auto merge of #725 - Susurrus:mman_constants, r=alexcrichton
Auto merge of #725 - Susurrus:mman_constants, r=alexcrichton

Add MAP_ANONYMOUS to DragonFlyBSD and FreeBSD

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