X509_cmp_time: only return 1, 0, -1.

The behaviour of X509_cmp_time used to be undocumented.

The new behaviour, documented in master, is to return only 0, 1, or -1.
Make the code in the other branches to adhere to this behaviour too,
to reduce confusion. There is nothing to be gained from returning
other values.

Fixes GH#4954

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4955)