Cygwin: pinfo: add method to send a serialized path_conv and HANDLE

Cygwin: pinfo: add method to send a serialized path_conv and HANDLE

To allow reopening a file open in another process by HANDLE, introduce
a matching file_pathconv method, taking a file descriptor as parameter.
The result is a serialized path_conv and a HANDLE value.  The HANDLE is
valid in the foreign process and MUST be duplicated into the target
process before usage.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>