MINOR: config: make tune.recv_enough configurable

MINOR: config: make tune.recv_enough configurable

This setting used to be assigned to a variable tunable from a constant
and for an unknown reason never made its way into the config parser.

tune.recv_enough <number>
  Haproxy uses some hints to detect that a short read indicates the end of the
  socket buffers. One of them is that a read returns more than <recv_enough>
  bytes, which defaults to 10136 (7 segments of 1448 each). This default value
  may be changed by this setting to better deal with workloads involving lots
  of short messages such as telnet or SSH sessions.