Support libcrypto for hmac and sha256 (#223)

Support libcrypto for hmac and sha256 (#223)

* compat: Add OpenSSL libcrypto compatibility layer

Detect libcrypto in configure script.  Only fall back
to using libcrypto when /usr libs are allowed and no
other compatible implementation is available or when
--with-openssl is passed explicitly.
Make sure libcrypto and libmd are never linked at the
same time.

Add OpenSSL based SHA256 and HMAC compat shims in
compat/crypt_openssl. Depeding on version and build flags,
libcrypto ships with a compatible SHA256 API in
"openssl/sha.h".  OpenSSL 3 has deprecated the SHA API,
so if it is not detected we fall back to an EVP_DIGEST
based version.
Because the API might still be in use in OpenSSL internally,
the compatibility wrappers have a dhcpcd_ prefix to avoid
symbol conflicts.

* Add sha256 tests based on the existing hmac-md5 tests.