Fix compat with libcurl 7.56.1+ and file:// wrapper

Fix compat with libcurl 7.56.1+ and file:// wrapper

Since 7.52.x libcurl file:// scheme was implemented in a way described
in https://tools.ietf.org/html/draft-ietf-appsawg-file-scheme-16 . The
draft is still not accepted and the change contained a BC breach with
win32 path handling. It was reported upstream and 7.52.x fixed it, but
the BC breaching behavior was reintroduced in 7.56.1. Thus, it is better
to handle this on the PHP side.