[maiko] with pcap, ethernet -E option handling should be improved

[nbriggs]

The maiko (lde/ldex/ldesdl) -E option allows an ethernet hardware address and/or an interface name to be specified. The pcap code requires knowing the interface name. The behavior should be

• neither name nor address known: choose the first non-loopback, up, running interface with an ethernet hardware address
• only name known: choose the ethernet hardware address from the named interface
• only ethernet hardware address known: choose the interface name which has that address
• both interface name and hardware address known: check for consistency

Assuming the ethernet was explicitly asked for, If anything fails (or is inconsistent), the code should exit with an error rather than proceeding with no network.