implement a catchall address for a domain

by specifying a "destination" in an account that is just "@" followed by the
domain, e.g. "@example.org". messages are only delivered to the catchall
address when no regular destination matches (taking the per-domain
catchall-separator and case-sensisitivity into account).

for issue #18

testdata/smtp/catchall/domains.conf

@@ -0,0 +1,12 @@
+Domains:
+	mox.example:
+		LocalpartCatchallSeparator: +
+Accounts:
+	mjl:
+		Domain: mox.example
+		Destinations:
+			mjl@mox.example: nil
+	catchall:
+		Domain: mox.example
+		Destinations:
+			@mox.example: nil