ring: prepare ring to allow new sync schemes

ring: prepare ring to allow new sync schemes

To make these preparations two main things are done:
- Change from *single* to *sync_type* to allow different
  synchronisation schemes to be applied.
  Mark *single* as deprecated in comments.
  Add new functions to allow user to query ring sync types.
  Replace direct access to *single* with appropriate function call.
- Move actual rte_ring and related structures definitions into a
  separate file: <rte_ring_core.h>. It allows to refer contents
  of <rte_ring_elem.h> from <rte_ring.h> without introducing a
  circular dependency.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>