Change "mox backup $destdir" from storing only data files to $destdir to storing those under $destdir/data and now also copying config files to $destdir/config. (#150)

Upgrade note: Admins may want to check their backup scripts.

Based on feedback in issue #150.

testdata/ctl/config/domains.conf

@@ -0,0 +1,12 @@
+Domains:
+	mox.example: nil
+Accounts:
+	mjl:
+		OutgoingWebhook:
+			URL: http://localhost:1234
+		KeepRetiredMessagePeriod: 1h0m0s
+		KeepRetiredWebhookPeriod: 1h0m0s
+		Domain: mox.example
+		Destinations:
+			mjl2@mox.example: nil
+			mjl@mox.example: nil

testdata/ctl/config/mox.conf

@@ -0,0 +1,9 @@
+DataDir: ../data
+User: 1000
+LogLevel: trace
+Hostname: mox.example
+Postmaster:
+	Account: mjl
+	Mailbox: postmaster
+Listeners:
+	local: nil