The firewall module could be improved if properties that set up a service
on a port included info (see Propellor.Info and Propellor.Types.Info)
about the port(s) used.

While currently the ports have to be explicitly listed:

	& Apache.installed
	& Firewall.installed
	& Firewall.addRule (Rule INPUT ACCEPT (Proto TCP :- Port 80))
	& Firewall.addRule (Rule INPUT ACCEPT (Proto TCP :- Port 443))

Instead the ports would be derived from the installed services.

	& Apache.installed
	& Firewall.installed

There could also be some combinators to adjust the exposed
ports of a property.

	& localOnly Apache.installed
	& exposedPorts [443,80] (Apt.serviceInstalledRunning "apache2")

Such port enformation is also going to be needed as a basis of
[[type_level_port_conflict_detection]]. --[[Joey]]
