nmis8, host_resources plugin needs to guard better against noSuchInst…

…ance

fix crash when dividing by 'noSuchInstance', i.e. zero; fix noise for
treating 'noSuchInstance' as number in some other places.

Candidate releae of NMIS8.6.8

Tagged for fix to SUPPORT-5428

nmis8, changed defaults for keep_event_history and installer

event purging rules down to 7 days, updateconfig unconditionally
patches that in, installer now asks if ok to disable keep_event_history
(default yes),
installer now also asks about ping timeout and escalation changes
only if required to

nmis 8, cisco_features plugin MUST NOT print to stdout unconditionall…

…y...

nmis8, support.pl typo

nmis8, fix for pingonly nodes being degraded b/c snmp down

the change to open-session-AND-test-it for polling failover caused sub collect() to  attempt
at session open (which doesn't do anything) but then the session test fails (as expected) and the node is
sent into snmp down state...

nnis8, updated version to 8.6.6G in prep for release