Add option to disable dropped packet logging

[geerlingguy]

For local dev environments (e.g. Drupal VM), it's a little annoying to have a bunch of 'Dropped by firewall' messages in syslog, so it would be nice to have an option to disable these messages through a firewall_log_dropped_connections variable.

This is most annoying when you're running Dropbox, and there are dozens of dropped connections to port 7500 (I think) on all running VMs on a local machine.

[geerlingguy]

I'm going to use the variable name firewall_log_dropped_packets to better match the actual functionality.