Make provider provider_init thread safe, and flag checking/setting too
provider_init() makes changes in the provider structure, and needs a
bit of protection to ensure that doesn't happen concurrently with race
conditions.
This also demands a bit of protection of the flags, since they are
bits and presumably occupy the same byte in memory.
Reviewed-by: Paul Dale <pauli@openssl.org> Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14354)