BoringSSL depends on the platform's locking APIs to make internal global
state thread-safe, including the PRNG. On some single-threaded embedded
platforms, locking APIs may not exist, so this dependency may be disabled
with a build flag.

Doing so means the consumer promises the library will never be used in any
multi-threaded address space. It causes BoringSSL to be globally thread-unsafe.
Setting it inappropriately will subtly and unpredictably corrupt memory and
leak secret keys.

Unfortunately, folks sometimes misinterpreted OPENSSL_NO_THREADS as skipping an
internal thread pool or disabling an optionally extra-thread-safe mode. This is
not and has never been the case. Rename it to
OPENSSL_NO_THREADS_CORRUPT_MEMORY_AND_LEAK_SECRETS_IF_THREADED to clarify what
this option does.

Update-Note: As a first step, this CL makes both OPENSSL_NO_THREADS and
OPENSSL_NO_THREADS_CORRUPT_MEMORY_AND_LEAK_SECRETS_IF_THREADED work. A later CL
will remove the old name, so migrate callers after or at the same time as
picking up this CL.

Change-Id: Ibe4964ae43eb7a52f08fd966fccb330c0cc11a8c
Reviewed-on: https://boringssl-review.googlesource.com/32084
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>