random-util: introduce RANDOM_DONT_DRAIN

Originally, the high_quality_required boolean argument controlled two
things: whether to extend any random data we successfully read with
pseudo-random data, and whether to return -ENODATA if we couldn't read
any data at all.

The boolean got replaced by RANDOM_EXTEND_WITH_PSEUDO, but this name
doesn't really cover the second part nicely. Moreover hiding both
changes of behaviour under a single flag is confusing. Hence, let's
split this part off under a new flag, and use it from random_bytes().

src/basic/random-util.h

@@ -8,6 +8,7 @@
 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 */
 } 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 */