Cache: fixed cache valid slot to reject incorrect statuses.

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.