Support `GET UPSDESC`

[Aram Peres]

https://networkupstools.org/docs/developer-guide.chunked/ar01s09.html

UPSDESC

Form:
	GET UPSDESC <upsname>
	GET UPSDESC su700

Response:
	UPSDESC <upsname> "<description>"
	UPSDESC su700 "Development box"

<description> is the value of "desc=" from ups.conf for this UPS. If it is not set, upsd will return "Unavailable".