fix login for account names with non-ascii chars

we include the username in session cookie values. but cookie values must be ascii-only, go's net/http's drops bad values. the typical solution is to querystring-encode/decode the cookie values, which we'll now do.

problem found by arnt, thanks for reporting!

testdata/httpaccount/domains.conf

@@ -1,11 +1,11 @@
 Domains:
 	mox.example: nil
 Accounts:
-	mjl:
+	mjl☺:
 		Domain: mox.example
 		FullName: mjl
 		Destinations:
-			mjl@mox.example:
+			mjl☺@mox.example:
 				Mailbox: Inbox
 				Rulesets:
 					-