Only process one epoll event at a time.

If ndhc were a high-performance program that handled lots of events,
this change would harm performance.  But it is not, and it implicitly
believes that events come in one at a time.  Processing batches
would make it harder to assure correctness while also never allocating
memory at runtime.

The previous structure was fine when everything was handled
immediately by callbacks, but it isn't now.

src/ndhc.c

@@ -263,14 +263,13 @@ static void fail_if_state_dir_dne(void)
         suicide("state_dir path '%s' does not specify a directory", state_dir);
 }
 
-#define NDHC_NUM_EP_FDS 7
 static void do_ndhc_work(void)
 {
     static bool rfkill_set; // Is the rfkill switch set?
     static bool rfkill_nl_state_changed; // iface state changed during rfkill
     static int rfkill_nl_state; // current iface state during rfkill
     struct dhcpmsg dhcp_packet;
-    struct epoll_event events[NDHC_NUM_EP_FDS];
+    struct epoll_event events[1];
     long long nowts;
     int timeout;