and implement OVERLAY() for bit strings and bytea.
In passing also convert text OVERLAY() to a true built-in, instead of
relying on a SQL function.
Leonardo F, reviewed by Kevin Grittner
to do that, but inconsistently.) Make bit type reject too short input,
too, per SQL. Since it no longer zero pads, 'zpbit*' has been renamed to
'bit*' in the source, hence initdb.