util: make size macros unsigned

util: make size macros unsigned

By making them unsigned comparing them with other sizes is less likely
to trigger compiler warnings regarding signed/unsigned comparisons.
After all sizes (i.e. size_t) are generally assumed to be unsigned, so
these should be too.

Prompted-by: https://github.com/systemd/systemd/pull/17345#issuecomment-709402332
(cherry picked from commit 67bd5620f6cf481c0a59cedbcf63ddcab355cc55)