random-util: initialize srand() from RDRAND

It's cheap to get RDRAND and given that srand() is anyway not really
useful for trusted randomness let's use RDRAND for it, after all we have
all the hard work for that already in place.

src/basic/random-util.c

@@ -144,6 +144,7 @@ void initialize_srand(void) {
 #if HAVE_SYS_AUXV_H
         const void *auxv;
 #endif
+        uint64_t k;
 
         if (srand_called)
                 return;
@@ -164,6 +165,9 @@ void initialize_srand(void) {
         x ^= (unsigned) now(CLOCK_REALTIME);
         x ^= (unsigned) gettid();
 
+        if (rdrand64(&k) >= 0)
+                x ^= (unsigned) k;
+
         srand(x);
         srand_called = true;
 }