Refactoring: struct lttng_event_field

- Namespace this structure with lttng_ust_ prefix,
- Use struct_size extensibility scheme,
- Remove padding,
- Use an array of pointers to structure rather than array of event
  fields to allow easier handling of extensibility. This is achieved by
  using __LTTNG_COMPOUND_LITERAL().
- Move nofilter field into structure.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Iaa0056ff2d67042d0b6fca156c8f8072ae1820ea