Force disable use of RDRAND in OpenSSL when HardwareAccel is enabled
FreeBSD announced direct use of RDRAND as sole entropy source is not recommended.
In Tor crypto_global_init() there is a call to ENGINE_load_builtin_engines() which lets OpenSSL take advantage of AES-NI acceleration. This is almost always A Good Thing.
From Sandy Bridge onward, however, this also results in the application using RDRAND directly for all entropy.
Since Tor cannot build the OpenSSL linked against (to set OPENSSL_NO_RDRAND), the workaround is to call RAND_set_rand_engine(NULL) after ENGINE_load_builtin_engines().
"FreeBSD Developer Summit: Security Working Group, /dev/random" https://wiki.freebsd.org/201309DevSummit/Security
"Surreptitiously Tampering with Computer Chips" https://www.schneier.com/blog/archives/2013/09/surreptitiously.html
"How does the NSA break SSL? ... Weak random number generators" http://blog.cryptographyengineering.com/2013/12/how-does-nsa-break-ssl.html