Change optional handling in ASN.1 encoder

Change optional handling in ASN.1 encoder

Create a new atype_optional with a function pointer to decide whether
the type is present in the C object.  For simple cases, sequences just
reference the optional version of a type.  For more complex cases (such
as when the presence of the usec field of a sequence depends on whether
the timestamp is set), we define a predicate on the structure object
and nest the field type inside the optional type.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25692 dc483132-0cff-0310-8789-dd5450dbe970