Create separate type for bit-fields.

Create separate type for bit-fields.

The definition of bit-fields with type OPJ_UINT32 caused complilation errors
on IBM iSeries, because OPJ_UINT32 is defined as uint32_t, and
uint32_t is defined as unsigned long in <stdint.h>. The definition of
bit-fields with an integer type of  a specific size doesn't make sense
anyway.