Fixed lingering_time check.

There are two significant changes in this patch:

1) The <= 0 comparison is done with a signed type.  This fixes the case
   of ngx_time() being larger than r->lingering_time.

2) Calculation of r->lingering_time - ngx_time() is now always done
   in the ngx_msec_t type.  This ensures the calculation is correct
   even if time_t is unsigned and differs in size from ngx_msec_t.

Thanks to Lanshun Zhou.