Fix NetworkChecker hangs on Linux (See #10824)

Rather than attempting to connect to o.org.uk from Linux clients,
we attempt to use the Java 1.6 classes via reflection. If the
client is running on Java 1.5, the network is always assumed to
be up. This may cause deadlocks on changing networks for
Linux+Java 1.5 users, which will have to be clearly outlined in
the release notes.

For the more far more common cases, however, this should be work.
Fix return value as seen by @jburel
Minor cleanup
Reduce verbose System.err as seen by @zeb