Core: improved ngx_reset_pool() (ticket #490).

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.