src/rrd_daemon.c: Use `sread' and `swrite' instead of read(2) and write(2).

src/rrd_daemon.c: Use `sread' and `swrite' instead of read(2) and write(2).

The ``secure'' sread assures that the received string ends with a newline,
strips it off the buffer and assures that the returned buffer is null
terminated. swrite returns success only if all bytes could be written.