Make many X509_xxx types opaque.

Make X509_OBJECT, X509_STORE_CTX, X509_STORE, X509_LOOKUP,
and X509_LOOKUP_METHOD opaque.
Remove unused X509_CERT_FILE_CTX

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Dr. Stephen Henson <steve@openssl.org>