- Add gw_monitor_tag(): derives InfluxDB tag from gateway PTR record
  (NAME.egw.etersoft.ru → NAME, with .v6 suffix for IPv6)
- find_gw_monitor: fallback to PTR-based tag when no monitor file
- Fix find_gw_monitor: use parse_gw_line instead of resolve_gw to
  handle "IP metric N" format in gateway files
- Fix build_route_via and load_list_routes multipath: use
  parse_gw_line loop instead of resolve_gw with raw gateway lines
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>