Disallow installing gosu with setuid

There are workarounds for this, but I will intentionally not be describing them because this is definitely not something I can endorse in any way.  Please don't use gosu in this way.

main.go

@@ -47,6 +47,13 @@ Usage: {{ .Self }} user-spec command [args]
 func main() {
 	log.SetFlags(0) // no timestamps on our logs
 
+	if fi, err := os.Stat("/proc/self/exe"); err != nil {
+		log.Fatalf("error: %v", err)
+	} else if fi.Mode()&os.ModeSetuid != 0 {
+		// ... oh no
+		log.Fatalf("error: %q appears to be installed with the 'setuid' bit set, which is an *extremely* insecure and completely unsupported configuration! (what you want instead is likely 'sudo' or 'su')", os.Args[0])
+	}
+
 	if len(os.Args) >= 2 {
 		switch os.Args[1] {
 		case "--help", "-h", "-?":