hddtemp: Support more than 32 drives

The hddtemp plugin imposes a 32-drive and 1024-byte limit on the response from the hddtemp daemon.  On my system, the 1024-byte limit truncates the response after 29 drives.

Drop the drive limit and increase the buffer limit to 1 MB.  (This is hopefully larger than anyone will need, and prevents collectd from being DoSed by the hddtemp daemon.)