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.
