SI-6026 REPL checks for javap before tools.jar

If javap is already available, don't go hunting for tools.jar

This avoids the getResource bug in AbstractFileClassLoader.

However, javap or tools.jar must still be provided for the workaround to work around.

Manual test:

```
apm@mara:~$ edge6 -toolcp /usr/lib/jvm/java-6-openjdk-amd64/lib/tools.jar
Welcome to Scala version 2.10.4-20131017-101001-371bad2fe6 (OpenJDK 64-Bit Server VM, Java 1.6.0_27).
Type in expressions to have them evaluated.
Type :help for more information.

scala> class Foo
defined class Foo

scala> :javap Foo
Compiled from "<console>"
public class Foo extends java.lang.Object{
    public Foo();
}
```