std: Clarify what timers do with zero and negative durations

Add tests. Also fix a bunch of broken time tests.

src/libstd/io/process.rs

@@ -976,7 +976,7 @@ mod tests {
                 assert!(!p.wait().unwrap().success());
                 return
             }
-            timer::sleep_ms(100);
+            timer::sleep(Duration::milliseconds(100));
         }
         fail!("never saw the child go away");
     })