Add missing ASN1_TIME functions

Do some cleanup of the ASN1_TIME code.
Add ASN1_TIME_normalize() to normalize ASN1_TIME structures.
Add ASN1_TIME_compare() to compare two ASN1_TIME structures.
Add ASN1_TIME_cmp_time_t() to compare an ASN1_TIME to time_t
(generic version of ASN1_UTCTIME_cmp_time_t()).

Replace '0' .. '9' compares with isdigit()
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2753)