lib/rules: log more info for a MDB_BAD_VALSIZE case

Vladimír Čunátrequested to merge
log-BAD_VALSIZE into master

Issue #956: it's not difficult to run into this limit by using blocking style with hosts-like files pointing names to localhost. Now it additionally explains a little, e.g.:

[rules ] records too big (512 B) for 1.0.0.127.in-addr.arpa. PTR

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