SECURITY: Make GETIL_B always have type `unsigned', not `int'

SECURITY: Make GETIL_B always have type `unsigned', not `int'

Otherwise unwanted sign extension can easily occur when converting
from the implicit int to unsigned long, eg in GET_L.

Without this, ttls and the 32-bit SOA fields can be sign-extended,
exposing invalid values to the application.

Found by AFL 2.35b.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>