Set the padding character for short, fixed-length records for the Queue
and Recno access methods.
If no pad character is specified, <space> characters (i.e.,
ASCII 0x20) are used for padding.
The Db::set_re_pad interface may only be used to configure Berkeley DB before
the Db::open interface is called.
The Db::set_re_pad method either returns a non-zero error value or throws an exception that
encapsulates a non-zero error value on failure, and returns 0 on success.