dovecot: Support dovecot-side username changes

Dovecot has options for changing the formatting of usernames; for
example, dropping the domain part, or replacing characters.

chasquid's implementation, however, fails to handle this well, as it
expects the reply to contain the username exactly as requested.

This patch fixes the problem by making chasquid ignoring the returned
username, which is unused anyway. The protocol is unambiguous enough.

Tests are also amended to always exercise this case.

internal/dovecot/dovecot.go

@@ -118,7 +118,7 @@ func (a *Auth) Exists(user string) (bool, error) {
 	resp, err := conn.ReadLine()
 	if err != nil {
 		return false, fmt.Errorf("error receiving response: %v", err)
-	} else if strings.HasPrefix(resp, "USER\t1\t"+user+"\t") {
+	} else if strings.HasPrefix(resp, "USER\t1\t") {
 		return true, nil
 	} else if strings.HasPrefix(resp, "NOTFOUND\t") {
 		return false, nil

test/t-11-dovecot/config/dovecot.conf.in

@@ -5,6 +5,10 @@ ssl = no
 default_internal_user = $USER
 default_login_user = $USER
 
+# Before auth checks, rename "u@d" to "u-AT-d". This exercises that chasquid
+# handles well the case where the returned user information does not match the
+# requested user.
+auth_username_format = "%n-AT-%d"
 
 passdb {
 	driver = passwd-file

test/t-11-dovecot/config/passwd

@@ -1 +1 @@
-user@srv:{plain}password:1000:1000::/home/user
+user-AT-srv:{plain}password:1000:1000::/home/user