aliases: Implement catch-all

This patch implements support for catch-all aliases, where users can add
a `*: destination` alias. Mails sent to unknown users (or other aliases)
will not be rejected, but sent to the indicated destination instead.

Please see https://github.com/albertito/chasquid/issues/23 and
https://github.com/albertito/chasquid/pull/24 for more discussion and
background.

Thanks to Alex Ellwein (aellwein@github) for the alternative patch and
help with testing; and to ThinkChaos (ThinkChaos@github) for help with
testing.

internal/smtpsrv/server.go

@@ -76,6 +76,8 @@ type Server struct {
 
 // NewServer returns a new empty Server.
 func NewServer() *Server {
+	authr := auth.NewAuthenticator()
+	aliasesR := aliases.NewResolver(authr.Exists)
 	return &Server{
 		addrs:          map[SocketMode][]string{},
 		listeners:      map[SocketMode][]net.Listener{},
@@ -83,8 +85,8 @@ func NewServer() *Server {
 		connTimeout:    20 * time.Minute,
 		commandTimeout: 1 * time.Minute,
 		localDomains:   &set.String{},
-		authr:          auth.NewAuthenticator(),
-		aliasesR:       aliases.NewResolver(),
+		authr:          authr,
+		aliasesR:       aliasesR,
 	}
 }