Fixes assertion failure in tests on NetBSD:
slow/prob_distr/stochastic_log_logistic: [forking] May 25 03:56:58.091 [err] tor_assertion_failed_(): Bug: src/lib/crypt_ops/crypto_rand_fast.c:184: crypto_fast_rng_new_from_seed: Assertion inherit != INHERIT_RES_KEEP failed; aborting. (on Tor 0.4.1.1-alpha-dev 29955f13e5)
May 25 03:56:58.091 [err] Bug: Assertion inherit != INHERIT_RES_KEEP failed in crypto_fast_rng_new_from_seed at src/lib/crypt_ops/crypto_rand_fast.c:184: . (Stack trace not available) (on Tor 0.4.1.1-alpha-dev 29955f13e5)
[Lost connection!]
This test was previously written to use the contents of the system
headers to decide whether INHERIT_NONE or INHERIT_ZERO was going to
work. But that won't work across different environments, such as
(for example) when the kernel doesn't match the headers. Instead,
we add a testing-only feature to the code to track which of these
options actually worked, and verify that it behaved as we expected.
Closes ticket 29541; bugfix not on any released version of Tor.
Using an anonymous mmap() is a good way to get pages that we can set
kernel-level flags on, like minherit() or madvise() or mlock().
We're going to use that so that we can make uninheritable locked
pages to store PRNG data.
