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/os_freebsd.odin

@@ -505,9 +505,21 @@ write_at :: proc(fd: Handle, data: []byte, offset: i64) -> (n: int, err: Error)
 }
 
 seek :: proc(fd: Handle, offset: i64, whence: int) -> (i64, Error) {
+	switch whence {
+	case SEEK_SET, SEEK_CUR, SEEK_END:
+		break
+	case:
+		return 0, .Invalid_Whence
+	}
 	res := _unix_seek(fd, offset, c.int(whence))
 	if res == -1 {
-		return -1, get_last_error()
+		errno := get_last_error()
+		switch errno {
+		case .EINVAL:
+			return 0, .Invalid_Offset
+		case:
+			return 0, errno
+		}
 	}
 	return res, nil
 }