Fix: agent thread poll set creation failure results in deadlock

Failing to initialize the agent thread's pollset will cause
the thread to exit before calling sessiond_notify_ready().

This will cause the main thread to wait forever for all threads
to be launched when such an error occurs.

The agent thread is not needed for the sessiond to work (except
to enable the tracing of Java and Python applications). Such
a failure should leave the sessiond in a useable state.

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