replace <tt></tt> in the javadoc comments by @code/@literal

Beginning with Java 9, javadoc default behaviour is to generate html5 javadoc and report errors if a doc comment if not html5 compatible. The markup tt is not part of the html5 spec so it has to be replaced by either @code or @literal (introduced in java 6, i believe).

I've used Eclipse search to pattern match all and replace them either with @literal if the content was true/false or null or with @code otherwise. I've then taken a look to the result to see/fix the remaning issues.