Fix: disable-event/channel return error if no domain

The disable-event/channel command now check for a UST/kernel session
before trying to disable the object. If none exists, it returns an error
without creating a domain.

Fixes #638

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: clarify man page lttng.1

Fixes #689

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: implicit conversion from enumeration

Fixes #759

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