NMS-6971 warn and disable ping if ICMP is unavailable
NMS-6971 add an option for disabling ICMP from the command-line
NMS-6971 only pop up the dialog if GUI is enabled
NMS-6971 better handling of ICMP failures in the remote poller

Merge remote-tracking branch 'origin/jira/NMS-6971' into rc/stable/1.14.0