Fix go test

In a user namespace, sending credentials for an unmapped user return
EINVAL instead of EPERM. So handle that case.

http://hydra.nixos.org/build/44839000
(cherry picked from commit 1dd0fb6b5a7c44d1b632466f936ca74268d13298)

pkgs/development/compilers/go/creds-test.patch

@@ -0,0 +1,14 @@
+diff -ru -x '*~' ./result/src/syscall/creds_test.go go-go1.7.4-src/src/syscall/creds_test.go
+--- ./result/src/syscall/creds_test.go	1970-01-01 01:00:01.000000000 +0100
++++ go-go1.7.4-src/src/syscall/creds_test.go	2016-12-21 14:06:39.559932164 +0100
+@@ -62,8 +62,8 @@
+ 		if sys, ok := err.(*os.SyscallError); ok {
+ 			err = sys.Err
+ 		}
+-		if err != syscall.EPERM {
+-			t.Fatalf("WriteMsgUnix failed with %v, want EPERM", err)
++		if err != syscall.EPERM && err != syscall.EINVAL {
++			t.Fatalf("WriteMsgUnix failed with %v, want EPERM or EINVAL", err)
+ 		}
+ 	}
+