The routine is mainly intended for use in conjunction with the
sla_DBJIN routine. If the value of JB indicates that an explicit
B or J prefix was detected by sla_DBJIN, a `B' or `J'
is returned to match. If JB indicates that no explicit B or J
was supplied, the choice is made on the basis of the epoch
itself; B is assumed for E <1984, otherwise J.