Auto merge of #859 - mwanner:freebsd-drop-drv, r=alexcrichton

Use IFF_OACTIVE and IFF_RUNNING even on FreeBSD. Deprecate the DRV ones.

According to @asomers, libc should propagate the use of the portable constants `IFF_OACTIVE` and `IFF_RUNNING` for user-space applications, instead of `IFF_DRV_OACTIVE` and `IFF_DRV_RUNNING`. It least that's my understanding of [his comment](https://github.com/nix-rust/nix/pull/667#discussion_r152396984) in nix-rust/nix#667.