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.

src/core/ngx_palloc.c

diff --git a/src/core/ngx_palloc.c b/src/core/ngx_palloc.c
index efbc244..1f70f9e 100644
--- a/src/core/ngx_palloc.c
+++ b/src/core/ngx_palloc.c
@@ -105,11 +105,14 @@
         }
     }
 
-    pool->large = NULL;
-
     for (p = pool; p; p = p->d.next) {
         p->d.last = (u_char *) p + sizeof(ngx_pool_t);
+        p->d.failed = 0;
     }
+
+    pool->current = pool;
+    pool->chain = NULL;
+    pool->large = NULL;
 }