Cache file format is recursive.  A "node" consists of

- optionally, a list of addresses:
	a	flag indicating addresses follow
	%u:	count N of addresses
	%lu:	timestamp in time_t format
	N repetitions of:
		%u:%s	length and contents of address
- a list of subnodes:
	%u:	count M of subnodes
	M repetitions of:
		%u:%s	length and contents of subnode's name
		node contents in this same format

The cache file is simply a node, representing the DNS root.

For example, if example.com has abuse@example.com as the only address,
cached at time_t 987654321, and no other nodes were cached:

1:3:com1:7:examplea1:987654321:17:abuse@example.com0: