Refactor ASN1_TIME_adj_internal()

Refactor ASN1_TIME_adj_internal()

ASN1_TIME_adj_internal() does some strange dances with remembering
allocations in a boolean and using strlen(p) to deduce what happened
inside *_string_from_tm(). It also (mis)translates a NULL p to an
illegal time value error.

This can be streamlined by converting directly from a struct tm into an
ASN1_TIME and setting the errors when they occur instead of trying to
deduce them from a NULL return. This is made a bit uglier than necessary
due to the reuse-or-allocate semantics of the public API.

At the cost of a little code duplication, ASN1_TIME_adj_internal()
becomes very easy and ASN1_TIME_to_generalizedtime() is also simplified
somewhat.

ok inoguchi jsing