Do no drop IP fragments (needed for some UDP VPN).

- Probably since SVN before R462 / git e4a934 (2012-03-01 15:46:22),
  all IP fragments were dropped.
- Clients normally shouldn't send fragments if MTUs are configured
  correctly, but IP allows to do so.
- Some UDP based VPNs may generate fragments, because VPN overhead
  increases packet size and the UDP tunnel packet can exceed MTU.
- Fixed a log_dbg() statement format string.