Quite a few improvements to SSDP server support:
common defines of used strings; fixed generation of ST/versus NT in SSDP responses;
fixed filtering on ssdp searches so matches case insenstively on target,
and supports ssdp:all properly

Renamed Advertisement ST field to Target (and use flag in serialze to say NT or ST).

UPnP::MungePrimaryMacAddrIntoBaseDeviceID() generates lower case hex digits (comply with suggestion
in spec).