Handle JDK8 -Xdoclint

The Oracle team have added the doclint tool to JDK 8. The tool validates Javadoc as part of a standard Javadoc run. Unfortunately, with the default settings, it rejects many HTML elements that are perfectly acceptable to browsers, and all invalid Javadoc references (@links). This is likely to prove very unpopular with developers.

Action needed:
1) Provide a maven-javadoc-plugin configuration item and property that can control the doclint tool (currently this requires using additionalparam AFAICT).

2) Apply the -Xdoclint:none option by default, so that doclint is opt-in, not opt-out (ie. fix Oracle's messed up default). This will also make it much easier for developers to handle migration to JDK 8.