Fix record_source() sometimes incorrectly returning regex.list

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Fix interchanged indices in dnsmasq.h. This bug resulted in sometimes having displayed regex.list even though the blocking was done due to a gravity.list entry. Although being of only cosmetic nature, this bug showed up every now and then and was never really traceable.

The problem appeared when more than one additional hosts file was loaded (e.g. gravity.list + black.list). As the first file would have gotten index 3 and the second 4, and record_source(4) returned the path of regex.list. This is now fixed by defining regex.list to ID 3 and start all additional HOSTS files from 4 onward.

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