Fix: Handle both agent config files pointing to same port

The expected locations for the user and root agent sessiond
configuration files are ~/.lttng/agent.port and
/var/run/lttng/agent.port, respectively. These files indicate
which port an agent should connect to to reach its respective
sessiond.

If by some bad luck both files indicate the same port, then
both Java TCP clients would end up connecting to the same
sessiond, resulting in weird results, like "lttng list" listing
all events twice.

Make sure the target ports are different, and avoid duplicate
connections in case there are not.

Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>