RDPDR file read/write may be truncated depending on platform

While the issue with 32-bit read/write operations has been largely addressed by GUACAMOLE-764, these read/write operations still rely on lseek(), which is not required to implement 64-bit seeks (off_t is not guaranteed to be 64-bit). Additional changes are likely needed to ensure that seeking within files works with 64-bit offsets in a cross-platform way.