Lower dnsmasq DHCP timeouts

I propose that we lower the timeout for dynamic ("!known") hosts from 6h to 3m. Basically, it is legal for a client to hold an assigned address even over reboots, so long dynamic timeouts can make it very hard to re-assign a mistaken address. [I believe this applies to either the MOXAs or to some of the Lantronix ports in the piepan]