Rework internal tm_to_*() converters

Rework internal tm_to_*() converters

Make them static. Don't make them allocate if passed a NULL ASN1_TIME to
avoid leaks. This currently means that we accept a NULL and succeed. That's
very ugly but better than what we have now.

Simplify ASN1_TIME_set_string_internal() accordingly and allocate an
ASN1_TIME at the API boundary of ASN1_TIME_adj_internal() and of
ASN1_TIME_to_generalized_time().

ok beck (after a lot of squealing and distress)