unix: fix or skip failing tests on android and iOS

The mkfifo syscall is disallowed on android and iOS. sched_setaffinity
is disallowed on android. Skip all tests which use them.

/usr/bin does not exist on android. Use /system/bin in TestGetwd
instead, like TestChdirAndGetwd in the os package.

Like linux, android does not support Fchmodat with flags != 0. Adjust
TestFchmodat accordingly.

TestDevices might stat some device files which are not accessible, skip
those.

iOS cannot exec subprocesses, thus skip TestPassFD.

Fixes golang/go#25535

Change-Id: Ic764b9152f0a7b703ad4f47fdb1a9a5e94718154
Reviewed-on: https://go-review.googlesource.com/114395
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/dev_linux_test.go

@@ -33,6 +33,9 @@ func TestDevices(t *testing.T) {
 			var stat unix.Stat_t
 			err := unix.Stat(tc.path, &stat)
 			if err != nil {
+				if err == unix.EACCES {
+					t.Skip("no permission to stat device, skipping test")
+				}
 				t.Errorf("failed to stat device: %v", err)
 				return
 			}