Fix Win32 portability problems

The network filter / snapshot / hooks code introduced some
non-portable pices that broke the win32 build

* configure.ac: Check for net/ethernet.h required by nwfile config
   parsing code
* src/conf/nwfilter_conf.c: Define ethernet protocol  constants
  if net/ethernet.h is missing
* src/util/hooks.c: Disable hooks build on Win32 since it lacks
  fork/exec/pipe
* src/util/threads-win32.c: Fix unchecked return value
* tools/virsh.c: Disable SIGPIPE on Win32 since it doesn't exist.
  Fix non-portable strftime() formats

tools/virsh.c

@@ -414,6 +414,7 @@ out:
  */
 static int disconnected = 0; /* we may have been disconnected */
 
+#ifdef SIGPIPE
 /*
  * vshCatchDisconnect:
  *
@@ -442,6 +443,10 @@ vshSetupSignals(void) {
 
     sigaction(SIGPIPE, &sig_action, NULL);
 }
+#else
+static void
+vshSetupSignals(void) {}
+#endif
 
 /*
  * vshReconnect:
@@ -8425,7 +8430,7 @@ cmdSnapshotList(vshControl *ctl, const vshCmd *cmd)
                              &creation) < 0)
                 continue;
             localtime_r(&creation, &time_info);
-            strftime(timestr, sizeof(timestr), "%F %T %z", &time_info);
+            strftime(timestr, sizeof(timestr), "%Y-%m-%d %H:%M:%S %z", &time_info);
 
             vshPrint(ctl, " %-20s %-25s %s\n", names[i], timestr, state);
         }