Implement issetugid syscall

Implement issetugid syscall and provide a test. This gets rid of the
scary "Unsecure. Implement me" warning during compilation.

lib/nbsd_libc/sys-minix/issetugid.c

@@ -6,6 +6,10 @@
 
 int issetugid(void)
 {
-#warning Unsecure. Implement me.
-	return 0;
+	int r;
+	message m;
+
+	r = _syscall(PM_PROC_NR, ISSETUGID, &m);
+	if (r == -1) return 0;	/* Default to old behavior */
+	return(r);
 }