Fix: don't set enabled flag is session start fails

The "started" var. is changed to the flag "has_been_started" indicating
if at least ONE start command has been seen.

The "enabled" var. is changed to the flag "active" and the semantic is
the same.

Backported from master.

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

Fixes #801

Conflicts:
	src/bin/lttng-sessiond/cmd.c