Add foldmultilines to UL for stdout/err

We've introduced `foldmultilines` to UL in JDK-8271186 to replace newline characters within a multiline log event with the character sequence '\' and 'n'. It helps to work log shippers.

`foldmultilines` supports file output only. It is useful if it affects to stdout/err output because many container applications would redirect logs to stdout.