Auto merge of rust-lang#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.

Author: bors

src/unix/bsd/freebsdlike/mod.rs

@@ -264,6 +264,8 @@ pub const EOF: ::c_int = -1;
 pub const SEEK_SET: ::c_int = 0;
 pub const SEEK_CUR: ::c_int = 1;
 pub const SEEK_END: ::c_int = 2;
+pub const SEEK_DATA: ::c_int = 3;
+pub const SEEK_HOLE: ::c_int = 4;
 pub const _IOFBF: ::c_int = 0;
 pub const _IONBF: ::c_int = 2;
 pub const _IOLBF: ::c_int = 1;