Merge with MySQL 5.1.55

- Fixed some issues with partitions and connection_string, which also fixed lp:716890 "Pre- and post-recovery crash in Aria"
- Fixed wrong assert in Aria

Now need to merge with latest xtradb before pushing 

sql/ha_partition.cc:
  Ensure that m_ordered_rec_buffer is not freed before close.
sql/mysqld.cc:
  Changed to use opt_stack_trace instead of opt_pstack.
  Removed references to pstack
sql/partition_element.h:
  Ensure that connect_string is initialized
storage/maria/ma_key_recover.c:
  Fixed wrong assert

sql/opt_range.h

@@ -271,6 +271,7 @@ public:
 
   virtual bool reverse_sorted() = 0;
   virtual bool unique_key_range() { return false; }
+  virtual bool clustered_pk_range() { return false; }
 
   enum {
     QS_TYPE_RANGE = 0,
@@ -541,6 +542,8 @@ public:
   THD *thd;
   int read_keys_and_merge();
 
+  bool clustered_pk_range() { return test(pk_quick_select); }
+
   /* used to get rows collected in Unique */
   READ_RECORD read_record;
 };