su: cleanery

* make help message more like others
* s/TT\.(.)Argu/TT.\1/g
* move environ to toys.h
* simplify failure messages
* clear password before quit
* not check what execve returns
* -lc
diff --git a/toys.h b/toys.h
index 2ce1d0a..c810123 100644
--- a/toys.h
+++ b/toys.h
@@ -117,6 +117,8 @@
 
 extern char toybuf[4096];
 
+extern char **environ;
+
 #define GLOBALS(...)
 
 #define ARRAY_LEN(array) (sizeof(array)/sizeof(*array))