CURR11 — Add interface to DTime

Summary: The Currency submission indicates that a specific Currency instance that
does not have an expiration date will be noted with an expiration date of
99/99/9999. This attribute is handled via a CBO::Dtime instance, but a
CBO::Dtime instance cannot be created or mutated to represent 99/99/9999.
e.g. The CBO::Dtime::setMonth(mon) expects 1 <= 12, etc. Would probably be
easiest to have two operations on CBO::Dtime to handle this. These could
be:
void setUnspecified() – might be handled in an implementation as
99/99/9999.
boolean isUnspecified() – insulates the consumer from knowing how
the notion of “unspecified” is implemented.