SHOW ALL PROBLEMS FOR MERGE TABLE COMPLIANCE IN 5.1".

The problem was that CHECK/REPAIR TABLE for a MERGE table which
had several children missing or in wrong engine reported only
issue with the first such table in its result-set. While in 5.0
this statement returned the whole list of problematic tables.

Ability to report problems for all children was lost during
significant refactorings of MERGE code which were done as part
of work on 5.1 and 5.5 releases.

This patch restores status quo ante refactorings by changing
code in such a way that:
1) Failure to open child table due to its absence during CHECK/
   REPAIR TABLE for a MERGE table is not reported immediately
   when its absence is discovered in open_tables(). Instead
   handling/error reporting in such a situation is postponed
   until the moment when children are attached.
2) Code performing attaching of children no longer stops when
   it encounters first problem with one of the children during
   CHECK/REPAIR TABLE. Instead it continues iteration through
   the child list until all problems caused by child absence/
   wrong engine are reported.

Note that even after this change problem with mismatch of
child/parent definition won't be reported if there is also
another child missing, but this is how it was in 5.0 as well.