Creation of EVP_MD_CTX and EVP_PKEY is quite expensive, until
we fix the code to reuse the context and key we'll use our own
implementation of siphash.
This commits adds an OpenSSL based isc_siphash24() implementation, which is
preferred when available.
The siphash_test has been modified to test both implementation with a trick that
renames the isc_siphash24() to openssl_ or native_ prefixed name and includes
the ../siphash.c two times (when the OpenSSL implementation is available).
The native implementation's conversion from the uint8_t buffers to uint64_t now
follows the reference implementation that doesn't require aligned buffers.
