Fix the checking of flags values in the settings tab

Changes:
- Accept setting an empty flags-type value in the settings tab
  if the variable specification permits it
- Don't accept substrings of flag values
  E.g. with values: 'one,two,three', 'hree', 'w', etc. used to
  be accepted. Not any more
- Don't accept flags with random pattern-matching special characters
  E.g. with values: 'one,two,three', 'on.', '(o)[n]e*' etc. used
  to be accepted. Not any more.

builtin/mainmenu/tab_settings.lua

@@ -506,8 +506,8 @@ local function handle_change_setting_buttons(this, fields)
 			local new_value = fields["te_setting_value"]
 			for _,value in ipairs(new_value:split(",", true)) do
 				value = value:trim()
-				if not value:match(CHAR_CLASSES.FLAGS .. "+")
-						or not setting.possible:match("[,]?" .. value .. "[,]?") then
+				local possible = "," .. setting.possible .. ","
+				if not possible:find("," .. value .. ",", 0, true) then
 					this.data.error_message = fgettext_ne("\"$1\" is not a valid flag.", value)
 					this.data.entered_text = fields["te_setting_value"]
 					core.update_formspec(this:get_formspec())