check_ntp_peer: Verify type of response packets

The check_ntp_peer plugin now ignores response packets with an
unexpected "Operation Code" and/or "Sequence" number (cf. RFC 1305,
Appendix B).  This fixes an issue where the check_ntp_peer requests got
duplicated on their way to the server (due to some network switch bug),
and the server therefore sent multiple responses per request.

Thanks to Marc Haber for reporting the issue.
Add libnpcommon.a file to .gitignore