It can now be shared by established callback-based resolvers and the new
VSS_ResolveOne. This also changes the semantics of VSS_ResolveOne in the
sense that the port is now a default port, overriden by the address if
it contains one.

Also make it clear that VTCP was already relying on a VSS function that
didn't and still doesn't allow a null errp argument, while conversely
all VTCP_open call sites provide a valid errp argument.