MDL-38558 add preupgrade script support

This is intended things like undoing of local hacks, it is similar to older pre 2.0 upgrade hack.

lib/upgradelib.php

@@ -1518,15 +1518,13 @@ function upgrade_core($version, $verbose) {
 
         print_upgrade_part_start('moodle', false, $verbose);
 
-        // one time special local migration pre 2.0 upgrade script
-        if ($CFG->version < 2007101600) {
-            $pre20upgradefile = "$CFG->dirroot/local/upgrade_pre20.php";
-            if (file_exists($pre20upgradefile)) {
-                set_time_limit(0);
-                require($pre20upgradefile);
-                // reset upgrade timeout to default
-                upgrade_set_timeout();
-            }
+        // Pre-upgrade scripts for local hack workarounds.
+        $preupgradefile = "$CFG->dirroot/local/preupgrade.php";
+        if (file_exists($preupgradefile)) {
+            set_time_limit(0);
+            require($preupgradefile);
+            // Reset upgrade timeout to default.
+            upgrade_set_timeout();
         }
 
         $result = xmldb_main_upgrade($CFG->version);

local/readme.txt

@@ -39,8 +39,8 @@ are supported. There may be some extra files with special meaning in /local/.
 Sample /local/ directory listing:
 /local/nicehack/         - first customisation plugin
 /local/otherhack/        - other customisation plugin
-/local/upgrade_pre20.php - one time upgrade and migration script which is
-                           executed before main 2.0 upgrade starts
+/local/preupgrade.php    - executed before each core upgrade, use $version and $CFG->version
+                           if you need to tweak specific local hacks
 /local/defaults.php      - custom admin setting defaults