Backport : request.getPathInfo() returns wrong path info in cms preview for submounts

When you for example create a submount 'sub' and then browse in cms preview to

http://localhost:8080/site/_cmsinternal/sub/about

you get

request.getPathInfo() --> "sinternal/sub"

It is better that _cmsinternal, just like _rp, _cmsrest etc is not part of the pathInfo but of the servletPath