Make write() and sendto() properly handle short writes and errors in ndhc.

Make ndhc write a pidfile.
Clean up defines.h and split out ifchd and ndhc specifics.

ifchd/ifchd-defines.h

@@ -0,0 +1,14 @@
+#ifndef IFCHD_DEFINES_H_
+#define IFCHD_DEFINES_H_
+
+#include "defines.h"
+
+#define PID_FILE_DEFAULT "/var/run/ifchd.pid"
+#define IFCHD_VERSION "0.8"
+#define COMM_SOCKET_PATH "ifchange"
+#define MAX_BUF 1024
+#define SOCK_QUEUE 2
+#define CONN_TIMEOUT 60
+
+#endif /* IFCHD_DEFINES_H_ */
+