SSC: Support for TX IRQ and other SSC updates (#552)

. Support TX IRQ for both TCP and COM modes (fixes #522)
. Support CTS/RTS (fixes #311)
. Fixed reading DIPSW registers
. Fixed TCP mode which was only generating an IRQ for first byte received
. Added UpdateCommandReg() to consolidate all updates to SSC command reg
. Updated reference URLs

source/SaveState.cpp

@@ -175,6 +175,7 @@ static void Snapshot_LoadState_v1()	// .aws v1.0.0.1, up to (and including) Appl
 		KeybReset();
 		VideoResetState();
 		MB_Reset();
+		sg_SSC.CommReset();
 
 		//
 		// Apple2 unit
@@ -497,6 +498,7 @@ static void Snapshot_LoadState_v2(void)
 		KeybReset();
 		VideoResetState();
 		MB_Reset();
+		sg_SSC.CommReset();
 #ifdef USE_SPEECH_API
 		g_Speech.Reset();
 #endif