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

effects/schema/pacman.schema.json

@@ -1,4 +1,4 @@
-{
+{
 	"type":"object",
 	"script" : "pacman.py",
 	"title":"edt_eff_pacman_header_title",