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.

mysql-test/r/merge.result:
  Added test case for bug #11754210 - "45777: CHECK TABLE DOESN'T
  SHOW ALL PROBLEMS FOR MERGE TABLE COMPLIANCE IN 5.1".
  Adjusted results of existing tests to the fact that CHECK/REPAIR
  TABLE statements now try to report problems about missing table/
  wrong engine for all underlying tables, and to the fact that
  mismatch of parent/child definitions is always reported as an
  error and not a warning.
mysql-test/t/merge.test:
  Added test case for bug #11754210 - "45777: CHECK TABLE DOESN'T
  SHOW ALL PROBLEMS FOR MERGE TABLE COMPLIANCE IN 5.1".
sql/sql_base.cc:
  Changed code responsible for opening tables to ignore the fact
  that underlying tables of a MERGE table are missing, if this
  table is opened for CHECK/REPAIR TABLE.
  The absence of underlying tables in this case is now detected and
  appropriate error is reported at the point when child tables are
  attached. At this point we can produce full list of problematic
  child tables/errors to be returned as part of CHECK/REPAIR TABLE
  result-set.
storage/myisammrg/ha_myisammrg.cc:
  Changed myisammrg_attach_children_callback() to handle new
  situation, when during CHECK/REPAIR TABLE we do not report 
  error about missing child immediately when this fact is 
  discovered during open_tables() but postpone error-reporting
  till the time when children are attached. 
  Also this callback is now responsible for pushing an error
  mentioning problematic child table to the list of errors to 
  be reported by CHECK/REPAIR TABLE statements.
  Finally, since now myrg_attach_children() no longer relies on
  return value from callback to determine the end of the children
  list, callback no longer needs to set my_errno value and can
  be simplified.
  
  Changed myrg_print_wrong_table() to always report a problem
  with child table as an error and not as a warning. This makes
  reporting for different types of issues with child tables
  more consistent and compatible with 5.0 behavior.
storage/myisammrg/myrg_open.c:
  Changed code in myrg_attach_children() not to abort on the
  first problem with a child table when attaching children to
  parent MERGE table during CHECK/REPAIR TABLE statement 
  execution. This allows CHECK/REPAIR TABLE to report problems 
  about absence/wrong engine for all underlying tables as
  part of their result-set.