The common construct seen in the BIND 9 source is func(isc_mem_t *mctx, ...).
Unfortunately, the dnstest.{h,c} has been using mctx as a global symbol, which
in turn generated a lot of errors when update.c got included in update_test.c.
As a rule of thumb, we should avoid naming global symbols with generic names
(like mctx) and we should prefix them with "namespace" (like dt_mctx).
The three functions has been modeled after the arc4random family of
functions, and they will always return random bytes.
The isc_random family of functions internally use these CSPRNG (if available):
1. getrandom() libc call (might be available on Linux and Solaris)
2. SYS_getrandom syscall (might be available on Linux, detected at runtime)
3. arc4random(), arc4random_buf() and arc4random_uniform() (available on BSDs and Mac OS X)
4. crypto library function:
4a. RAND_bytes in case OpenSSL
4b. pkcs_C_GenerateRandom() in case PKCS#11 library