MQDLH_* (Dead-letter header structure)

See:

MQDLH_STRUC_ID "DLHb"
MQDLH_STRUC_ID_ARRAY 'D','L','H','b'
MQDLH_VERSION_1 1 X'00000001'
MQDLH_CURRENT_VERSION 1 X'00000001'