Add support for ajc -xlintfile

As noted in the ajc reference documentation, the compiler supports an -xlintfile option that allows fine grained control over the level that compiler messages are logged. Without this option you can only set the default level for ALL messages the compiler can generate, which isn't very useful if you just want to enforce a single rule. For example, I would like to set the adviceDidNotMatch messages to an error while leaving all others the same. Setting the global options increases items that might have normally been totally ignored to now be an error that will fail your build.