chore: initial import from KNEL/PFVCluster@041d311 [#769]

Split per O&M lane work order. Full history: KNEL/PFVCluster.
shellcheck debt fixed in librenms-agent (quoting, arithmetic, backticks);
fixes upstream ntp-server.sh bug: input_wakeups reported PACKETSENDFAILURES
instead of INPUTWAKEUPS. mysql.sh (PHP-in-.sh) exempt per house rule.

https://projects.knownelement.com/issues/769#note-4152

librenms-agent/agent/check_mk@.service

@@ -0,0 +1,7 @@
+[Unit]
+Description=Check_MK LibreNMS Agent Service
+After=network.target
+ 
+[Service]
+ExecStart=/usr/bin/check_mk_agent
+StandardOutput=socket