add UDP retries

[oh2kku]

Added a retry feature for UDP mode. This allows more than one UDP probe to be sent before returning a critical status to Nagios. The feature can be used to prevent (low) packet loss from showing up as a problem in a given OpenVPN server in Nagios UI and event logs.

Example command lines:

• send up to 5 packets with 1 second between each packet, maximum total time around 5 seconds: check_openvpn 192.0.2.1 --timeout 1 --retrycount 5
• send up to 4 packets with 3 seconds between each packet, maximum total time around 12 seconds: check_openvpn 192.0.2.1 --timeout 3 --retrycount 4

Default timeout has not been changed and default retrycount is 1 to preserve the same behavior as previous versions. The timeout should typically be set lower than default when retries are used.

[andiwand]

that's very useful, thank you!

just a small remark: in python you usually write the for loop like: for i in range(retrycount):

[liquidat]

That's a great contribution - thank you very much!