packets are lost; do not send the events if a node has requested
bootinfo within the last 10 seconds; this implies that the bootinfo
reply packet was lost and the node is retrying. This should quiet
things a bit from stated, although eventually this should be dealt
with in stated instead of with this hack.

Mac originally added the cache to proxydhcp. I've added a cache to
bootinfo to solve the same problem (proxydhcp is no longer used),
although I have changed to implementation to use the DBM backend
rather then a one-off hash table. dbopen() with a NULL file argument
creates an in-memory database (no backing store), and I have used the
DB_HASH db type. So, the hash table is done with a couple of calls to
DBM routines.