Report Invalid_Whence on some os platforms

- Move `Seek`-related checks into OS-specific files for granularity.

Platforms:
- Darwin
- FreeBSD
- Haiku
- Linux
- NetBSD
- OpenBSD

core/os/stream.odin

@@ -47,14 +47,6 @@ _file_stream_proc :: proc(stream_data: rawptr, mode: io.Stream_Mode, p: []byte,
 		}
 	case .Seek:
 		n, os_err = seek(fd, offset, int(whence))
-		if os_err != nil {
-			switch whence {
-			case .Start, .Current, .End:
-				return 0, .Invalid_Offset
-			case:
-				return 0, .Invalid_Whence
-			}
-		}
 	case .Size:
 		n, os_err = file_size(fd)
 	case .Destroy: