Make `javac` silent on warnings

It is typically uncommon for people to encounter warnings of the kind:

warning: Unsafe is internal proprietary API and may be removed in a future release

But mucking around with Java’s classlib internals is one of the things I get to do aplenty. To shut the warnings up (make it ignore ct.sym) use -XDignore.symbol.file`.

To suppress the warnings generated for the -Xlint types, either code the annotation @SuppressWarnings, or from the command line, use -Xlint:none or -nowarn (they are the same).