dns_list_base, 2 bug fixes: race condition in multi

and returning invalid results (three args instead of two) in lookup callback. Resulted in 'a' being not-null when it should have been.

pending was being incremented within multi/zones.forEach. That functions callback was returning and being executed before pending was incremented for the next RBL. Net effect, the dnsbl plugin saw its callback called with pending=0 for every DNSBL listed.
dnsbl: add error handler
dns_list_base: handle ENOTFOUND error
dnsbl: added search feature (multi -vs- first)
Merge branch 'dnsbl' into toaster

Conflicts:
	config/dnsbl.ini
	plugins/dnsbl.js