Improve auth. provider interface

The authentication provider can now provide multiple authorization
identities associated with credentials. Protocols that support that
(e.g. JMAP, SASL) can let the client select the wanted identity.

internal/module/auth.go

@@ -1,7 +1,16 @@
 package module
 
-// AuthProvider is the interface implemented by modules providing authentication using
+import "errors"
+
+var (
+	// ErrUnknownCredentials should be returned by auth. provider if supplied
+	// credentials are valid for it but are not recognized (e.g. not found in
+	// used DB).
+	ErrUnknownCredentials = errors.New("unknown credentials")
+)
+
+// PlainAuth is the interface implemented by modules providing authentication using
 // username:password pairs.
-type AuthProvider interface {
-	CheckPlain(username, password string) bool
+type PlainAuth interface {
+	AuthPlain(username, password string) ([]string, error)
 }