Change JUL agent to use the new ABI

Two things have changed, the agent port file created by the session
daemon, the name is now "agent.port". The second thing is that upon
registration, the agent needs to send its domain which is here
LTTNG_DOMAIN_JUL (val: 3).

Signed-off-by: David Goulet <dgoulet@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>