Add flag to avoid setting/creating user on login

Fixes roundcube#9377 . Allows rcmail::login() to be reused in plugins providing
an API. The login function provides some useful logic for connecting to
IMAP sources like checking credentials for validity or converting
usernames in some cases.

Before this change the login function always also created non-existing
users or set some sesison vars, which what API plugins would like to
avoid.

program/include/rcmail.php

@@ -647,14 +647,15 @@ public function session_init()
      * Perform login to the mail server and to the webmail service.
      * This will also create a new user entry if auto_create_user is configured.
      *
-     * @param string $username    Mail storage (IMAP) user name
-     * @param string $password    Mail storage (IMAP) password
-     * @param string $host        Mail storage (IMAP) host
-     * @param bool   $cookiecheck Enables cookie check
+     * @param string $username     Mail storage (IMAP) user name
+     * @param string $password     Mail storage (IMAP) password
+     * @param string $host         Mail storage (IMAP) host
+     * @param bool   $cookiecheck  Enables cookie check
+     * @param bool   $just_connect Breaks after successful connect
      *
      * @return bool True on success, False on failure
      */
-    public function login($username, $password, $host = null, $cookiecheck = false)
+    public function login($username, $password, $host = null, $cookiecheck = false, $just_connect = false)
     {
         $this->login_error = null;
 
@@ -773,6 +774,8 @@ public function login($username, $password, $host = null, $cookiecheck = false)
             return false;
         }
 
+        if ($just_connect) return true;
+
         // user already registered -> update user's record
         if (is_object($user)) {
             // update last login timestamp