dcttech-usbrelay: accept conn=sn=<serno> user specs in probe

Add support for the "-d dcttech-usbrelay:conn=sn=<serno>" syntax.

This dramatically increases usability when multiple cards are attached
(a very likely condition given the low channel count of these devices),
while other means to select one card out of a set of cards either don't
exist or are heavily constrained: Identical VID:PID numbers are shared
across all card variants. HIDAPI paths are rather platform dependent and
may even be incompatible with the conn= syntax. Some platforms may even
refuse to open() paths which result from their own enumeration. Which
leaves relay card serial numbers as the only reliable and portable
method for selection. Being convenient is just a bonus.