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.


git-svn-id: https://develop.svn.wordpress.org/trunk@45611 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/custom-header.php

@@ -778,7 +778,7 @@ endif;
 		}
 
 		if ( file_exists( $file ) ) {
-			list( $width, $height, $type, $attr ) = getimagesize( $file );
+			list( $width, $height, $type, $attr ) = @getimagesize( $file );
 		} else {
 			$data   = wp_get_attachment_metadata( $attachment_id );
 			$height = isset( $data['height'] ) ? $data['height'] : 0;