Fix: JUL filtering done on the UST level

This is to support enabling all events with different loglevels in two
different sessions.

For this, if any loglevel have been defined, the 'int_loglevel' filter
is added to the UST event. The liblttng-ust-jul library has been
modified to stop filtering loglevel in the agent.

This commit adds two tests, one for a back to back session that are
destroyed and a second one for multi loglevel session.

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: move JUL filter creation to a function

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: handle ENOENT when creating a kernel event

The kernel tracer returns ENOENT if an event is not found thus warn the
user of that.

Fixes #761

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: incorrect printf format

Fixes #777

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: Copy-pasted PERROR messages may be misleading

Some PERRORs in lttng-sessiond's pthread_create's error handling
code indicate the wrong thread name which may be misleading
during debugging.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>