Enhance dummy-ups repeater mode to multiplex several connections to the same device

[jimklimov]

Reminder from discussion with @aquette - dummy-ups can be a proxy to one other driver.

A value-added improvement could be to proxy several drivers (e.g. connections to the same ups via several medias or protocols) to get the best of non-overlapping supported abilities (e.g. different snmp mibs vs. NetXML) as well as redundancy (net vs. USB/serial).