Fix off-by-one error.

Make sure we follow the same rules at all places so we get coherent
results. Fix 1px border sometime being eaten by the background of a div,
as in http://whatif.xkcd.com.
Remove B_PROT_* and related code.

These custom error codes are going away from Network Kit soon. Use
standard ones instead.

Warning: this can't work with the previous versions of the network kit.