Core: improved ngx_reset_pool() (ticket #490). Previously pool->current wasn't moved back to pool, resulting in blocks not used for further allocations if pool->current was already moved at the time of ngx_reset_pool(). Additionally, to preserve logic of moving pool->current, the p->d.failed counters are now properly cleared. While here, pool->chain is also cleared. This change is essentially a nop with current code, but generally improves things.