auth: Implement an Authenticator type

This patch implements an Authenticator type, which connections use to
do authentication and user existence checks.

It simplifies the abstractions (the server doesn't need to know about
userdb, or keep track of domain-userdb maps), and lays the foundation
for other types of authentication backends which will come in later
patches.

internal/userdb/userdb_test.go

@@ -129,7 +129,7 @@ func TestWrite(t *testing.T) {
 
 	db = mustLoad(t, fname)
 	for _, name := range []string{"user1", "ñoño"} {
-		if !db.HasUser(name) {
+		if !db.Exists(name) {
 			t.Errorf("user %q not in database", name)
 		}
 		if db.db.Users[name].GetScheme() == nil {
@@ -294,12 +294,12 @@ func TestRemoveUser(t *testing.T) {
 	}
 }
 
-func TestHasUser(t *testing.T) {
+func TestExists(t *testing.T) {
 	fname := mustCreateDB(t, "")
 	defer removeIfSuccessful(t, fname)
 	db := mustLoad(t, fname)
 
-	if db.HasUser("unknown") {
+	if db.Exists("unknown") {
 		t.Errorf("unknown user exists")
 	}
 
@@ -307,15 +307,15 @@ func TestHasUser(t *testing.T) {
 		t.Fatalf("error adding user: %v", err)
 	}
 
-	if db.HasUser("unknown") {
+	if db.Exists("unknown") {
 		t.Errorf("unknown user exists")
 	}
 
-	if !db.HasUser("user") {
+	if !db.Exists("user") {
 		t.Errorf("known user does not exist")
 	}
 
-	if !db.HasUser("user") {
+	if !db.Exists("user") {
 		t.Errorf("known user does not exist")
 	}
 }