MINOR: config: implement global setting tune.buffers.reserve

MINOR: config: implement global setting tune.buffers.reserve

Used in conjunction with the dynamic buffer allocator.

tune.buffers.reserve <number>
  Sets the number of buffers which are pre-allocated and reserved for use only
  during memory shortage conditions resulting in failed memory allocations. The
  minimum value is 2 and is also the default. There is no reason a user would
  want to change this value, it's mostly aimed at haproxy core developers.