Refactoring: struct lttng_enum_desc and lttng_enum_entry

- Namespace these structures with lttng_ust_ prefix,
- Use struct_size extensibility scheme,
- Remove padding,
- Use an array of pointers to structure rather than array of enumeration
  entries to allow easier handling of extensibility. This is achieved by
  using __LTTNG_COMPOUND_LITERAL().

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I2bf6e24e22c9646fc321f6c7c9ffcbd32ed22da0