Bootstrap: Enhance core's memory limit handling.

* Don't lower memory limit if the current limit is greater than `WP_MAX_MEMORY_LIMIT`.
* Set `WP_MEMORY_LIMIT` and `WP_MAX_MEMORY_LIMIT` to current limit if the `memory_limit` setting can't be changed at runtime.
* Use `wp_convert_hr_to_bytes()` when parsing the value of the `memory_limit` setting because it can be a shorthand or an integer value.
* Introduce `wp_raise_memory_limit( $context )` to raise the PHP memory limit for memory intensive processes. This DRYs up some logic and includes the existing `admin_memory_limit` and `image_memory_limit` filters. The function can also be used for custom contexts, the `{$context}_memory_limit` filter allows to customize the limit.
* Introduce `wp_is_ini_value_changeable( $setting )` to determine whether a PHP ini value is changeable at runtime.
* Remove a `function_exists( 'memory_get_usage' )` check. Since PHP 5.2.1 support for memory limit is always enabled.

Related commits: [38011-38013]

Props jrf, A5hleyRich, swissspidy, ocean90.
Fixes #32075.
Built from https://develop.svn.wordpress.org/trunk@38015


git-svn-id: http://core.svn.wordpress.org/trunk@37956 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/admin.php

@@ -138,21 +138,7 @@ else
 	require(ABSPATH . 'wp-admin/menu.php');
 
 if ( current_user_can( 'manage_options' ) ) {
-	/**
-	 * Filters the maximum memory limit available for administration screens.
-	 *
-	 * This only applies to administrators, who may require more memory for tasks like updates.
-	 * Memory limits when processing images (uploaded or edited by users of any role) are
-	 * handled separately.
-	 *
-	 * The WP_MAX_MEMORY_LIMIT constant specifically defines the maximum memory limit available
-	 * when in the administration back end. The default is 256M, or 256 megabytes of memory.
-	 *
-	 * @since 3.0.0
-	 *
-	 * @param string 'WP_MAX_MEMORY_LIMIT' The maximum WordPress memory limit. Default 256M.
-	 */
-	@ini_set( 'memory_limit', apply_filters( 'admin_memory_limit', WP_MAX_MEMORY_LIMIT ) );
+	wp_raise_memory_limit( 'admin' );
 }
 
 /**