openvpn: reworked status file parser

openvpn: reworked status file parser

 * Added detection of status fields index by parsing 'HEADER' lines.
   This allows to support status files from latest OpenVPN-2.4 and possible from future versions;
 * Fixed a bug with OpenVPN status file version 3 where delimiter is 'tab' (not space or comma);
 * Status file version detection was moved from configuration callback to read callback;
 * Now 'plugin_register_complex_read()' is used for registering read callback.

Closes: #2128