MDL-40475 add alternative component cache location and other improvements

Improvements include:
* Alternative location might be useful when server administrator wants to maintain
  a local copy of component cache instead of using shared $CFG->cachedir.
* Component caching is now enabled in behat tests which should improve performance.
* Standardised ignoring of component caching.
* Fixed debug mode in ABORT_AFTER_CONFIG scripts.
* General documentation improvements.

admin/cli/install.php

@@ -139,6 +139,8 @@ define('CACHE_DISABLE_ALL', true);
 
 define('PHPUNIT_TEST', false);
 
+define('IGNORE_COMPONENT_CACHE', true);
+
 // Check that PHP is of a sufficient version
 if (version_compare(phpversion(), "5.3.3") < 0) {
     $phpversion = phpversion();