kill: Fix free() with bad pointer on SIG-prefixed signal-name

kill -l SIGHUP (or any other signal-name prefixed with "SIG")
would cause free() to be called with a bad pointer instead of
a pointer to what was allocated. Fix this and add test-case.

lib/signals.c

@@ -263,7 +263,7 @@ char *strtosig(const char *restrict s)
             }
         }
     }
-    free(p);
+    free(copy);
     return converted;
 }