Provide function in rcube to connect to IMAP

This is useful for plugins like FreeBusy [1] that previously
connected to IMAP via plain rcube_imap::connect().

[1] https://git.kolab.org/source/freebusy/browse/master/lib/Kolab/FreeBusy/SourceIMAP.php

program/include/rcmail.php

@@ -764,13 +764,7 @@ public function login($username, $password, $host = null, $cookiecheck = false,
         $storage = $this->get_storage();
 
         // try to log in
-        if (!$storage->connect($host, $username, $password, $port, $ssl)) {
-            if ($user) {
-                $user->failed_login();
-            }
-
-            // Wait a second to slow down brute-force attacks (#1490549)
-            sleep(1);
+        if (!$this->imap_connect($storage, $host, $username, $password, $port, $ssl, $user)) {
             return false;
         }
 

program/lib/Roundcube/rcube.php

@@ -1857,6 +1857,41 @@ public function deliver_message($message, $from, $mailto, &$error,
 
         return $sent;
     }
+
+    /**
+     * Helper method to establish connection to an IMAP backend.
+     *
+     * @param rcube_storage $imap         IMAP storage handler
+     * @param string        $host         IMAP host
+     * @param string        $username     IMAP username
+     * @param string        $password     IMAP password
+     * @param int           $port         IMAP port to connect to
+     * @param string        $ssl          SSL schema or false if plain connection
+     * @param rcube_user    $user         Roundcube user (if it already exists)
+     * @param array         $imap_options Additional IMAP options
+     *
+     * @return bool Return true on successful login.
+     */
+    public function imap_connect($imap, $host, $username, $password, $port, $ssl, $user = null, $imap_options = array())
+    {
+        // enable proxy authentication
+        if (!empty($imap_options)) {
+            $imap->set_options($imap_options);
+        }
+
+        // try to log in
+        if (!$imap->connect($host, $username, $password, $port, $ssl)) {
+            if ($user) {
+                $user->failed_login();
+            }
+
+            // Wait a second to slow down brute-force attacks (#1490549)
+            sleep(1);
+            return false;
+        }
+
+        return true;
+    }
 }
 
 /**