Collate lttng_enable_event* code

Because the function lttng_enable_event_with_exclusions() can also
handle the duties of lttng_enable_event and lttng_enable_event_with_filter
as special cases, there's no need to have the same code duplicated in
each function. Let the other two functions fall the generic function
lttng_enable_event_with_exclusions() instead.