Fix: return EINVAL if agent registration fails

The errno value might be 0 thus not returning an error if so. It has
been seen with an unstable python agent code base which means it could
happen in the future if a third part decides to create an agent.

Signed-off-by: David Goulet <dgoulet@efficios.com>
Add Python agent support

Support the new Python agent shipped in liblttng-ust.

This adds the -p, --python option to the list and enable/disable-event
command to control the domain exactly like JUL and LOG4J.

The agent support is for the Python "logging" module.

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