[#2419] Mark sun.nio.ch as optional

Motivation:

If sun.nio.ch is not optional this will cause troubles in the
OSGi world. The package is not exposed by default in OSGi, so
actually the whole netty framework cannot be used directly.

There are workarounds, but workarounds are ugly. Especially since
the use of sun.nio.ch is optional. So the requirement on the
package should be optional as well.

Modifications:

Make the import of sun.nio.ch optional.

Result:

If the package cannot be imported it will behave as if the package
sun.nio.ch is not present (like with other JVMs). If the package is
exposed in OSGi (e.g. bootclassloader delegation, extension fragment)
it will be used.