Access union value_data members consistently

Use the same, appropriate union value_data member for each access of
BOOLEAN, BYTE and SHORT PW_TYPEs, without assuming they're
interchangeable with "integer", as that is only true on little-endian
architectures.

This fixes at least this wimax unit test failure on s390x and ppc64:

    Mismatch in line 11 of src/tests/unit/wimax.txt, got: 1a 0c 00 00 60 b5 01 06 00 02 03 00 expected: 1a 0c 00 00 60 b5 01 06 00 02 03 01
Prefix *_ENDIAN macros with RADIUS_

Rename LITTLE_ENDIAN and BIG_ENDIAN macros to RADIUS_LITTLE_ENDIAN and
RADIUS_BIG_ENDIAN respectively to avoid clashes with
/usr/include/endian.h defines, which result in always assuming
little-endian architecture.
Don't assume little-endian in fr_prints_uint128

Add handling of big-endian architectures to fr_prints_uint128.