Review request on the optional modules support

[Mandy Chung]

I modified the code per Mark's review comments.

Updated webrev at:
   http://cr.openjdk.java.net/~mchung/jigsaw/webrevs/optional-modules.02/

* Move isModulePresent and requireModulePresent methods
   to ModuleClassLoader.  Also add the convenience methods
   in java.lang.Class.

* Class.getClassLoader for system classes returns the boot
   loader in module mode and returns null in legacy mode.

* Removed BootLoader.isModulePresent override.  The base and boot
   modules are in the same context and such override is not needed.

Mandy