Post-probing filters

Post-probing filters are required to identify whether a plugin can handle a given modem, in special cases where the information retrieved from udev is either not enough or wrong. This covers, for example, RS232 modems connected through a RS232 to USB converter, where udev-reported vendor ID is that of the converter, not the one of the modem.

Plugins which require post-probing filters will always be sorted last, and therefore they will be the last ones being checked for pre-probing filters. This is due to the fact that we need to assume that these plugins aren't able to determine port support just with pre-probing filters, as we want to avoid unnecessary probing sequences launched. Also note that the Generic plugin is anyway always the last one in the list.