[HCL] MicroDowell B.Box LP 500: genericups type 7

Closes networkupstools/nut#83

From @lxp:

UPS shutdown only works when on-battery and has a delay of about 1min until
execution (something between 50sec to 1min 30sec on mine).

References:
http://www.ezdirect.it/pdf/lp500.pdf
https://forums.gentoo.org/viewtopic-t-730172-start-0.html