Fix problem with domains associated with DHCP hosts at startup.

At startup, the leases file is read by lease_init(), and
in lease_init() undecorated hostnames are expanded into
FQDNs by adding the domain associated with the address
of the lease.

lease_init() happens relavtively early in the startup, party because
if it calls  the dhcp-lease helper script, we don't want that to inherit
a load of sensitive file descriptors. This has implications if domains
are defined using the --domain=example.com,eth0 format since it's long
before we call enumerate_interfaces(), so get_domain fails for such domains.

The patch just moves the hostname expansion function to a seperate
subroutine that gets called later, after enumerate_interfaces().

src/dnsmasq.h

@@ -1605,6 +1605,7 @@ void lease_update_from_configs(void);
 int do_script_run(time_t now);
 void rerun_scripts(void);
 void lease_find_interfaces(time_t now);
+void lease_calc_fqdns(void);
 #ifdef HAVE_SCRIPT
 void lease_add_extradata(struct dhcp_lease *lease, unsigned char *data, 
 			 unsigned int len, int delim);