cygwin: add cygwin specific normcase logic in cygwin environment, mount point part of path is treated as case sensitive, even though underlying NTFS is case insensitive. this patch preserves mount point part of specified path, only if it is absolute one. there is no easy way to get list of current mount points from python program, other than to execute "mount" external command, because cygwin does not store current mount points into Unix/Linux like /etc/XXXtab file. so, this patch introduces cygwinmountpoints variable to list mount points to be preserved case. this allows some other extensions to customize mount point configuration.