ntopng checks MySQL for the value of open_files_limit and engages
and alert if the worst case number of open files is greater
than or equal to the 80% of open_files_limit
Interfaces were handled globally before this commit. Now
alerts can be configured per-interface. So for example alerts
for the same host 193.168.2.1 can be handled differently on multiple interfaces.
Old alerts configured should be automatically migrated thanks to script startup.lua
This commits also paves the way to introduce stateful alerts in the professional version