WPS: Handle Selected Registrar as a union of info from all Registrars

WPS: Handle Selected Registrar as a union of info from all Registrars

Instead of using the latest selected registrar change, collect selected
registrar information separately from all registrars and use the union
of this information when building the WPS IE for Beacon and Probe
Response frames.

Note: SetSelectedRegistrar UPnP action does not include a unique
identifier, so the ER matching routine is based only on the IP address
of the ER. In theory, there could be multiple ERs using the same IP
address (but different port or URL), so there may be some corner cases
that would not always match the correct ER entry at the AP. Anyway, this
is not really expected to occur in normal use cases and even if it did
happen, the selected registrar information is not any worse than it was
before when only the last change from any registrar for being
advertized.