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

testdata/webmail/domains.conf

@@ -2,6 +2,9 @@ Domains:
 	disabled.example:
 		Disabled: true
 	mox.example:
+		LocalpartCatchallSeparators:
+			- +
+			- -
 		DKIM:
 			Selectors:
 				testsel: