dco_linux: clean up PEER_GET trigger and parser

dco_linux: clean up PEER_GET trigger and parser

This patch is intended to reduce code duplication and
cleanup the DCO code around the PEER_GET command.

Specifically it:
* unified PEER_GET reply parser for `multi` and
  `non-multi` case
* unified PEER_GET request trigger for `multi` and
  `non-multi` case
* dropped struct multi_context from the argument list of
  dco_get_peer_stats_multi()

Github: closes OpenVPN/openvpn#800
Change-Id: Icbc70225d53ca678b8c22ed437b424c16e199d66
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>