fix compiler warnings

fix compiler warnings

multiple warnings like

src/socket-proxy/socket-proxyd.c: In function ‘transfer_data_cb’:
src/socket-proxy/socket-proxyd.c:237:25: warning: format ‘%ld’ expects argument of type ‘long int’, but argument 6 has type ‘size_t’ [-Wformat=]
                         log_debug("Buffer now has %ld bytes full.", c->buffer_filled_len);