BIGNUM signed add/sub routines refactory

Old code replaced in favor of a clearer implementation.
Performances are not penalized.

Updated the copyright end date to 2018.

Reviewed-by: David Benjamin <davidben@google.com>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5963)