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-includes/load.php

@@ -1002,3 +1002,27 @@ function wp_convert_hr_to_bytes( $value ) {
 	// Deal with large (float) values which run into the maximum integer size.
 	return min( $bytes, PHP_INT_MAX );
 }
+
+/**
+ * Determines whether a PHP ini value is changeable at runtime.
+ *
+ * @since 4.6.0
+ *
+ * @link http://php.net/manual/en/function.ini-get-all.php
+ *
+ * @param string $setting The name of the ini setting to check.
+ * @return bool True if the value is changeable at runtime. False otherwise.
+ */
+function wp_is_ini_value_changeable( $setting ) {
+	static $ini_all;
+
+	if ( ! isset( $ini_all ) ) {
+		$ini_all = ini_get_all();
+	}
+
+	if ( isset( $ini_all[ $setting ]['access'] ) && ( INI_ALL === $ini_all[ $setting ]['access'] || INI_USER === $ini_all[ $setting ]['access'] ) ) {
+		return true;
+	}
+
+	return false;
+}