The file lib/dns/mapapi defines a format version for map files; it's
supposed to be changed whenever a structure that would be part of a map
file is changed (e.g., dns_rbtdb, dns_rbt, dns_rbtnode, rdatasetheader,
etc).

The format version is correctly written out into the map file header,
but it isn't checked at load time.  (We do check pointer size and endianness
and the per-node and per-file CRC values, but not this.)

This suggests the possibility of map files loading that shouldn't load,
which might cause assertion failures and other misbehavior.