Rename `enum lttng_enabler_type` to `_format_type`

This will avoid confusion between the different types of enablers
(event notifier enablers and event enablers).

- Enabler format types describe the way the event name matching is done.

- Enabler types will describe the type of enablers (event
  notifier vs event)

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ic71d05159c5f244d0b1ad74f9c0ee6247fcdfbbb