Speed up interface carrier checking.

This is done by performing one synchronous query for carrier state at
the start of the program; after that, we just monitor the nlsocket for
carrier state changes and update the cached state accordingly.

The benefit is that ifchd needs to do a lot less work and this should
reduce the CPU cycle consumption; prior to this commit, the CPU time
ends up being a few CPU-minutes per month.

src/ifchange.c

@@ -224,7 +224,7 @@ static int ifchwrite(const char buf[static 1], size_t count)
     return -1;
 }
 
-bool carrier_isup(void)
+bool ifchange_carrier_isup(void)
 {
     char buf[256];
     snprintf(buf, sizeof buf, "carrier:;");

src/ifchange.h

@@ -31,7 +31,7 @@
 
 #include <stdbool.h>
 
-bool carrier_isup(void);
+bool ifchange_carrier_isup(void);
 int ifchange_bind(struct client_state_t cs[static 1],
                   struct dhcpmsg packet[static 1]);
 int ifchange_deconfig(struct client_state_t cs[static 1]);

src/ndhc.c

@@ -110,6 +110,8 @@ int signals_flagged(void)
     return SIGNAL_NONE;
 }
 
+bool carrier_isup(void) { return cs.carrier_up; }
+
 void set_client_addr(const char v[static 1]) { cs.clientAddr = inet_addr(v); }
 
 void print_version(void)
@@ -311,6 +313,7 @@ static void do_ndhc_work(void)
         if (pfds[0].revents & POLLIN) {
             had_event = true;
             sev_nl = nl_event_get(&cs);
+            cs.carrier_up = (sev_nl == IFS_UP);
         }
         if (pfds[0].revents & (POLLHUP|POLLERR|POLLRDHUP)) {
             suicide("nlfd closed unexpectedly");
@@ -494,6 +497,7 @@ static void ndhc_main(void) {
     memset(chroot_dir, '\0', sizeof chroot_dir);
     nk_set_uidgid(ndhc_uid, ndhc_gid, (const unsigned char *)0, 0);
 
+    cs.carrier_up = ifchange_carrier_isup();
     if (!carrier_isup()) {
         if (ifchange_deconfig(&cs) < 0)
             suicide("%s: can't deconfigure interface settings", __func__);

src/ndhc.h

@@ -61,7 +61,7 @@ struct client_state_t {
     enum arp_state server_arp_state, router_arp_state;
     enum fprint_state fp_state;
     bool using_dhcp_bpf, arp_is_defense, check_fingerprint, program_init,
-         sent_renew_or_rebind;
+         sent_renew_or_rebind, carrier_up;
     bool sent_gw_query, sent_first_announce, sent_second_announce;
 };
 
@@ -99,6 +99,7 @@ extern uid_t ndhc_uid;
 extern gid_t ndhc_gid;
 
 int signals_flagged(void);
+bool carrier_isup(void);
 void set_client_addr(const char v[static 1]);
 void show_usage(void);
 void signal_exit(int status);