MDL-20948 proper removal of cached pasword hashes just in case upgrading from <1.9.7

auth/imap/db/install.php

@@ -0,0 +1,11 @@
+<?php
+
+function xmldb_auth_imap_install() {
+    global $CFG, $DB;
+
+    // upgrade from 1.9.x, introducing version.php
+
+    // remove cached passwords, we do not need them for this plugin
+    $DB->set_field('user', 'password', 'not cached', array('auth'=>'imap'));
+
+}

auth/imap/version.php

@@ -0,0 +1,3 @@
+<?php
+
+$plugin->version = 2009112400;