Convert generalizedtime_to_tm() to ASN1_TIME_to_tm()

Convert generalizedtime_to_tm() to ASN1_TIME_to_tm()

Second step of moving away from ASN1_time_parse(). Being an OpenSSL API,
ASN1_TIME_to_tm() supports a variety of things. In this specific case we
don't really want it to parse anything but a GeneralizedTime expressed in
Zulu time. Unfortunately, OpenSSL make this annoying. So punt on this and
only do checks for the correct type and length. LibreSSL only accepts Zulu
time, so there is no change of behavior.

ok claudio job