Part#1: Add optimizer_switch for DuplicateWeedout strategy.

Change the join order optimization so that it also possible to turn
off DuplicateWeedout semi-join strategy, and add an optimizer_switch
to use to turn it on/off.  Since for some join orders,
DuplicateWeedout may be the only possible strategy, we allow for it to
be selected even if strategy is turned off.  However, if we later find
a join order that support an enabled strategy, we switch to this even
if it is more expensive than the join order with DuplicateWeedout.
For this to work, we will have to do some changes to how plans are
pruned during greedy search.  See below for details.

sql/sql_const.h
sql/sys_vars.cc
  Add optimizer_switch duplicateweedout

sql/sql_planner.h
  Add Optimize_table_order::found_plan_with_allowed_sj.
  If DuplicateWeedout is disabled, this will be set to false until a
  plan without DuplicateWeedout has been found.

sql/sql_planner.cc
  Optimize_table_order::greedy_search
    Initialize found_plan_without_dupsweedout.  It is not sufficient to set
    it in constructor since if greedy search is done in several iterations,
    due to setting of optimizer_search_depth, it needs to be reset for each
    iteration.
  Optimize_table_order::consider_plan
    If DuplicateWeedout is disabled, check if current join order
    requires this strategy.
    If it does, only select it if it is cheaper than current best plan AND
    no plan without DuplicateWeedout has been found.
    If it does not, select it if is cheaper than current best plan OR
    no plan without DuplicateWeedout has been found.
    Added a line to Optimizer Trace to explain why a plan that is not
    cheaper is still selected.
  Optimize_table_order::best_extension_by_limited_search
    If DuplicateWeedout strategy is disabled, and current selected plan
    use DuplicateWeedout, do not prune other plans.
  Optimize_table_order::advance_sj_state
    If DuplicateWeedout strategy is disabled, do not select it when
    there are other applicable strategies for this join order.

mysql-test/t/subquery_sj_firstmatch.test
mysql-test/t/subquery_sj_loosescan.test
mysql-test/t/subquery_sj_mat.test
    Use new optimizer_switch to turn off DuplicateWeedout strategy for
    these tests.

mysql-test/r/subquery_sj_firstmatch.result
mysql-test/r/subquery_sj_firstmatch_bka.result
mysql-test/r/subquery_sj_firstmatch_bka_nixbnl.result
mysql-test/r/subquery_sj_firstmatch_bkaunique.result
    No plans with DuplicateWeedout anymore

mysql-test/r/subquery_sj_loosescan.result
mysql-test/r/subquery_sj_loosescan_bka.result
mysql-test/r/subquery_sj_loosescan_bka_nixbnl.result
mysql-test/r/subquery_sj_loosescan_bkaunique.result
mysql-test/r/subquery_sj_mat.result
mysql-test/r/subquery_sj_mat_bka.result
mysql-test/r/subquery_sj_mat_bka_nixbnl.result
mysql-test/r/subquery_sj_mat_bkaunique.result
    Less plans with DuplicateWeedout, but some plans still use
    DuplicateWeedout since LooseScan and Materialization is not
    applicable for all queries.  Note that plans that still use
    DuplicateWeedout may have changed to a different plan using
    DuplicateWeedout since pruning is no longer done in such cases and
    a cheaper plan may now be found.

mysql-test/suite/opt_trace/include/subquery.inc
mysql-test/suite/opt_trace/r/subquery_no_prot.result
mysql-test/suite/opt_trace/r/subquery_ps_prot.result
   Also turns off DuplicateWeedout for a test case.  Test now trace
   LooseScan which seems to be the original intention of the test.  It
   also shows the line added to optimizer trace when one switch to a
   more expensive plan because the previous plan used a disabled semijoin
   strategy.

mysql-test/r/index_merge_myisam.result
mysql-test/r/mysqld--help-notwin.result
mysql-test/r/mysqld--help-win.result
mysql-test/r/optimizer_switch.result
mysql-test/suite/sys_vars/r/optimizer_switch_basic.result
    Changes in result since output of optimizer_switch now contains an
    additional switch.