workers: Make sure to wait for worker processes to exit so that they don't become zombies.

--
MOS_MIGRATED_REVID=103541217
diff --git a/src/main/java/com/google/devtools/build/lib/shell/Command.java b/src/main/java/com/google/devtools/build/lib/shell/Command.java
index 364373d..b0ed4a5 100644
--- a/src/main/java/com/google/devtools/build/lib/shell/Command.java
+++ b/src/main/java/com/google/devtools/build/lib/shell/Command.java
@@ -846,8 +846,7 @@
         }
       }
     } finally {
-      // Read this for detailed explanation:
-      // http://www-128.ibm.com/developerworks/java/library/j-jtp05236.html
+      // Read this for detailed explanation: http://www.ibm.com/developerworks/library/j-jtp05236/
       if (wasInterrupted) {
         Thread.currentThread().interrupt(); // preserve interrupted status
       }