Cache: fixed cache valid slot to reject incorrect statuses. Previously, result of ngx_atoi() was assigned to an ngx_uint_t variable, and errors reported by ngx_atoi() became positive, so the following check in "status < 100" failed to catch them. This resulted in the configurations like "proxy_cache_valid 2xx 30s" being accepted as correct, while they in fact do nothing. Changing type to ngx_int_t fixes this, and such configurations are now properly rejected.