Split into multiple files

For multiple reasons the program is now split:
- Make future porting to OpenBSD easier
- Assign header includes to individiual functions
- Make future program extensions easier
- Recompile only changed parts

datetime.c

@@ -0,0 +1,15 @@
+#include <time.h>
+
+#include "util.h"
+
+const char *
+datetime(const char *fmt)
+{
+	time_t t;
+
+	t = time(NULL);
+	if (strftime(buf, sizeof(buf), fmt, localtime(&t)) == 0)
+		return NULL;
+
+	return buf;
+}