Fix: add some sane default values to --live opt

The lttng create --live now takes optional arguments meaning if no USEC
or/and -U are found, some defaults value are used.

Fixes #738

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: Warn user if JUL port is already being used

Fixes #740

Signed-off-by: David Goulet <dgoulet@efficios.com>