Auto merge of #723 - ncaracci:master, r=alexcrichton

Add SEEK_DATA and SEEK_HOLE constants to FreeBSD and DragonFlyBSD

They can be found [here](https://github.com/DragonFlyBSD/DragonFlyBSD/blob/725edadf86d63f56a584adf23265845c8590d734/sys/sys/unistd.h#L126) for DragonFlyBSD and [here](https://github.com/freebsd/freebsd/blob/f5d95e1f8d32db4ccccfd5ad9cecb21ed07a695d/sys/sys/unistd.h) for FreeBSD.