catacomb.c, util.c: Publish `KERR_...' symbols as negative numbers.

catacomb.c, util.c: Publish `KERR_...' symbols as negative numbers.

They're supposed to be, but the existing `setconstants' machinery isn't
clever enough to do this properly.

Fixing this involves adding a flags word to `struct nameval' to identify
which constants should really be signed, and some fancy footwork to
convert unsigned integers back to negative values without upsetting C.