Port disable vs. ignore

As of r1946, ports may be disabled and ignored the same as devices. In this context, disabled means disabled from Observium's perspective, not the device's perspective. Disabled means do not poll the port, and ignored means do not alert about the port - just like devices.

The main benefit of this feature is for edge switches (access layer switches in Cisco-speak). Their ports are constantly going up & down, and we don't want to be notified about this, but if the port is up, we want to gather statistics. So in this case, the port would be enabled, but ignored.

The naming of disabled and ignored is a little counter-intuitive, and clashes with the naming standards used by many switch manufacturers (where disabling a port means to make it administratively down). If an elegant upgrade path can be found, the features may be renamed to "poll" and "notify" (or something similar), where both settings would default to true.