unix: fix TestFchmodat on Illumos

On Illumos, TestFchmodat fails with:

--- FAIL: TestFchmodat (0.00s)
        syscall_unix_test.go:502: Fchmodat: unexpected error: operation not supported on transport endpoint

Like Linux, Illumos doesn't support flags != 0 in Fchmodat, see
https://illumos.org/man/2/chmod

Adjust TestFchmodat accordingly to handle EOPNOTSUPP on Illumos.

Change-Id: Icd4564497a41c4aa962cd76604b5ca2c7575d96c
Reviewed-on: https://go-review.googlesource.com/101775
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_unix_test.go

@@ -492,16 +492,18 @@ func TestFchmodat(t *testing.T) {
 	}
 
 	mode = os.FileMode(0644)
+	didChmodSymlink := true
 	err = unix.Fchmodat(unix.AT_FDCWD, "symlink1", uint32(mode), unix.AT_SYMLINK_NOFOLLOW)
 	if err != nil {
-		if runtime.GOOS == "linux" && err == unix.EOPNOTSUPP {
-			// Linux doesn't support flags != 0
+		if (runtime.GOOS == "linux" || runtime.GOOS == "solaris") && err == unix.EOPNOTSUPP {
+			// Linux and Illumos don't support flags != 0
+			didChmodSymlink = false
 		} else {
 			t.Fatalf("Fchmodat: unexpected error: %v", err)
 		}
 	}
 
-	if runtime.GOOS == "linux" {
+	if !didChmodSymlink {
 		// Didn't change mode of the symlink. On Linux, the permissions
 		// of a symbolic link are always 0777 according to symlink(7)
 		mode = os.FileMode(0777)