zzip: format: explicitly mark header length constants as unsigned

zzip: format: explicitly mark header length constants as unsigned

The various different `headerlength` macros in "format.h" are defined as
simple additions of all the field lengths of its struct representation.
While it is obvious that the resulting constants are always positive,
the C language specifies the result as a signed integer, causing
warnings at several places due to comparisons with different signedness.

Fix the issue by explicitly marking the constants as unsigned. A simpler
fix would just replace those constants with `sizeof`, which should be
fine due to the struct's being marked as packed. But for the sake of
simplicitly, let's just stay with the more obvious fix.