It implements an idea to save dhcp hosts' macaddr vs. ipaddr mappings to
static file and make dnsmasq loading it with "--dhcp-hostsfile" option,
originally suggested by Dan, and can address the problem that too
many "--dhcp-host" args hitting ARG_MAX limit

* src/util/dnsmasq.h src/util/dnsmasq.c: adds the 2 new files