Implement os.Executable

For now this is a stub for everything but linux, which is a slightly
modified copy of the official implementation.

Should address #1778.

src/os/executable_other.go

@@ -0,0 +1,9 @@
+// +build !linux
+
+package os
+
+import "errors"
+
+func Executable() (string, error) {
+	return "", errors.New("Executable not implemented")
+}