Add padding to EVP_Cipher

[robgjansen]

Rather than forcing aes_cbc_cipher using OPENSSL_ia32cap=~0x200000200000000 so that we don't have to deal with padding in EVP_Cipher, we could just implement the padding scheme.

See the mailing list thread starting here:
shadow/shadow#638

More info here:

• Disable TLS encryption in Scallion shadow#136

• shadow-plugin-tor/src/tor/shadowtor-preload.c

  Lines 99 to 120 in 064c460

  	* There is a corner case on certain machines that causes padding-related errors
  	* when the EVP_Cipher is set to use aesni_cbc_hmac_sha1_cipher. Our memmove
  	* implementation does not handle padding.
  	*
  	* We attempt to disable the use of aesni_cbc_hmac_sha1_cipher with the environment
  	* variable OPENSSL_ia32cap=~0x200000200000000, and by default intercept EVP_Cipher
  	* in order to skip the encryption.
  	*
  	* If that doesn't work, the user can request that we let the application perform
  	* the encryption by defining SHADOW_ENABLE_EVPCIPHER, which means we will not
  	* intercept EVP_Cipher and instead let OpenSSL do its thing.
  	*/
  	#ifndef SHADOW_ENABLE_EVPCIPHER
  	/*
  	* EVP_CIPHER_CTX *ctx
  	* The ctx parameter has been voided to avoid requiring Openssl headers
  	*/
  	int EVP_Cipher(struct evp_cipher_ctx_st* ctx, unsigned char *out, const unsigned char *in, unsigned int inl){
  	memmove(out, in, (size_t)inl);
  	return 1;
  	}
  	#endif

Thanks @rwails!