Allow multiple localpart catch all separators, e.g. both "+" and "-", for addresses you+anything@example.com and you-anything@example.com

The original config option stays, and we still use it for the common case where
we have a single separator. The "+" is configured by default. It is optional,
just like the new option "LocalpartCatchallSeparators" (plural).

When parsing the config file, we combine LocalpartCatchallSeparator and
LocalpartCatchallSeparators into a single list
LocalpartCatchallSeparatorsEffective, which we use throughout the code.

For issue #301 by janc13

webmail/api.json

@@ -2002,9 +2002,10 @@
 			"Docs": "DomainAddressConfig has the address (localpart) configuration for a domain, so\nthe webmail client can decide if an address matches the addresses of the\naccount.",
 			"Fields": [
 				{
-					"Name": "LocalpartCatchallSeparator",
+					"Name": "LocalpartCatchallSeparators",
 					"Docs": "Can be empty.",
 					"Typewords": [
+						"[]",
 						"string"
 					]
 				},