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

entropy.c

@@ -0,0 +1,12 @@
+#include <stdio.h>
+
+#include "util.h"
+
+const char *
+entropy(void)
+{
+	int num;
+
+	return (pscanf("/proc/sys/kernel/random/entropy_avail", "%d", &num) == 1) ?
+	               bprintf("%d", num) : NULL;
+}