hardware protocol v2: make the hello handshake idempotent

A device which doesn't hear the welcome repeats its greeting. Minting a
fresh session for every repeat left both ends chasing each other's
session ids: the device eventually answered with the id it had been
given, the simulator had already moved on and rejected it as
INVALID_SESSION, the device restarted the handshake, and so on twice a
second for as long as the link was up.

The same greeting arriving on a session which has not been used yet is
now answered with the session it already has, and only a different
device, or one whose session had progressed, gets a new id. Repeated
greetings are counted as handshakes and shown in the debug panel, so the
condition which causes them stays visible instead of flooding the log.

application/driveruipanels.cpp

@@ -1393,7 +1393,8 @@ debug_panel::update_section_hardware( std::vector<text_line> &Output ) {
             + "  symbol " + std::to_string( protocol.unknown_symbols )
             + "  handle " + std::to_string( protocol.unknown_handles )
             + "  duplicates " + std::to_string( protocol.duplicate_commands )
-            + "  rate limited " + std::to_string( protocol.rate_limited );
+            + "  rate limited " + std::to_string( protocol.rate_limited )
+            + "  handshakes " + std::to_string( protocol.handshakes );
 
         textline +=
             "\nack " + std::to_string( protocol.acks )