random-util: optionally allow randomness to be generated via RDRAND

We only use this when we don't require the best randomness. The primary
usecase for this is UUID generation, as this means we don't drain
randomness from the kernel pool for them. Since UUIDs are usually not
secrets RDRAND should be goot enough for them to avoid real-life
collisions.

src/basic/random-util.h

@@ -9,6 +9,7 @@ typedef enum RandomFlags {
         RANDOM_EXTEND_WITH_PSEUDO = 1 << 0, /* If we can't get enough genuine randomness, but some, fill up the rest with pseudo-randomness */
         RANDOM_BLOCK              = 1 << 1, /* Rather block than return crap randomness (only if the kernel supports that) */
         RANDOM_DONT_DRAIN         = 1 << 2, /* If we can't get any randomness at all, return early with -EAGAIN */
+        RANDOM_ALLOW_RDRAND       = 1 << 3, /* Allow usage of the CPU RNG */
 } RandomFlags;
 
 int genuine_random_bytes(void *p, size_t n, RandomFlags flags); /* returns "genuine" randomness, optionally filled upwith pseudo random, if not enough is available */