improve serial hotplug (#389)

* - disable device when error indecates that the problem is not solvable on reconnect
- introduce a preOpenDelay of 2 seconds (currently value is hardcoded)

* rs232:
- make preOpenDelay available via webui
- fix preOpenDelay
- add basic usb serial detection

* - revert 3819ae7
- fix schema files

* make json checks compat with utf8+bom

* make shutdown effect a bit more flexible

test/jsonchecks/checkjson.py

@@ -11,10 +11,10 @@ for filename in sys.argv[1:]:
 		total += 1
 		msg = "   check json %s ... " % filename
 		try:
-			json.load(f)
+			json.loads(f.read().decode('utf-8-sig'))
 			#print(msg + "ok")
 		except ValueError as e:
-			print(msg + 'invalid')
+			print(msg + 'invalid ('+str(e)+')')
 			retval = 1
 			errors += 1