Stop taking detour through unit_types[SECTORS] to find

Stop taking detour through unit_types[SECTORS] to find
dl.d_secsize.

Leave unit_types[SECTORS].ut_conversion at 0, and test that to
determine if a size needs to be converted from a sectors value.

Use consistent dance to find the desired size value to print.

Logic is clearer, unit_types[] is now const, nobody but misc.c
knows about SECTORS.

No intentional functional change.