internal: add renameat syscall on FreeBSD

Change-Id: I694fa828c6f71941412afbb46535eb40dbecb4b2

internal/renameat/renameat.go

@@ -3,6 +3,7 @@ package renameat
 // Renameat is a wrapper around renameat syscall.
 // On Linux, it is a wrapper around renameat2(2).
 // On Darwin, it is a wrapper around renameatx_np(2).
+// On FreeBSD, it is a wrapper around renameat(2).
 func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string, flags uint) (err error) {
 	return renameat(olddirfd, oldpath, newdirfd, newpath, flags)
 }

internal/renameat/renameat_freebsd.go

+package renameat
+
+import "golang.org/x/sys/unix"
+
+const (
+	// Since FreeBSD does not currently privode renameat syscall
+	// beyond POSIX standard like Linux and Darwin do, we borrow
+	// the defination from Linux but reject these non-POSIX flags.
+	RENAME_EXCHANGE = (1 << 1)
+)
+
+func renameat(olddirfd int, oldpath string, newdirfd int, newpath string, flags uint) (err error) {
+	if flags != 0 {
+		return unix.ENOSYS
+	}
+	return unix.Renameat(olddirfd, oldpath, newdirfd, newpath)
+}