Coding Standards: Fix instances of WordPress.PHP.NoSilencedErrors.Discouraged.

Noteable changes:
- The `magic_quotes_runtime` and `magic_quotes_sybase` settings were removed in PHP 5.4, so no longer need to be set.
- Some functions that use external libraries can generate errors that can't be tested for, so are globally allowed to silence errors.
- Quite a few functions would cause errors if `safe_mode` was set. This setting was removed in PHP 5.4.
- Only a handful of `header()` calls needed corresponding `headers_sent()` checks for unit tests to pass, but more may need to be added as the nightlies builds are tested.

See #46732.

Built from https://develop.svn.wordpress.org/trunk@45611


git-svn-id: http://core.svn.wordpress.org/trunk@45422 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/plugin.php

@@ -376,7 +376,7 @@ function get_mu_plugins() {
 		return $wp_plugins;
 	}
 
-	@closedir( $plugins_dir );
+	closedir( $plugins_dir );
 
 	if ( empty( $plugin_files ) ) {
 		return $wp_plugins;
@@ -444,7 +444,7 @@ function get_dropins() {
 		return $dropins;
 	}
 
-	@closedir( $plugins_dir );
+	closedir( $plugins_dir );
 
 	if ( empty( $plugin_files ) ) {
 		return $dropins;