Follow-up for the fix for bug #46947 "Embedded SELECT without

FOR UPDATE is causing a lock".
 
This patch tries to address problems which were exposed 
during backporting of original patch to 5.1 tree.
 
- It ensures that we don't change locking behavior of simple
  SELECT statements on InnoDB tables when they are executed
  under LOCK TABLES ... READ and with @@innodb_table_locks=0.
  Also we no longer pass TL_READ_DEFAULT/TL_WRITE_DEFAULT 
  lock types, which are supposed to be parser-only, to 
  handler::start_stmt() method.
- It makes check_/no_concurrent_insert.inc auxiliary scripts 
  more robust against changes in test cases that use them
  and also ensures that they don't unnecessarily change 
  environment of caller.

mysql-test/include/check_concurrent_insert.inc:
  Reset DEBUG_SYNC facility before and after using it in
  auxiliary script. This makes this script more robust against
  changes in test cases calling it. It also ensures that script
  does not unnecessarily change environment of caller.
mysql-test/include/check_no_concurrent_insert.inc:
  Reset DEBUG_SYNC facility before and after using it in
  auxiliary script. This makes this script more robust against
  changes in test cases calling it. It also ensures that script
  does not unnecessarily change environment of caller.
mysql-test/r/innodb-lock.result:
  Added coverage for LOCK TABLES ... READ behavior in
  @@innodb_table_locks = 0 mode. This test also checks
  that an appropriate type of lock is passed to
  handler::start_stmt() method.
mysql-test/t/innodb-lock.test:
  Added coverage for LOCK TABLES ... READ behavior in
  @@innodb_table_locks = 0 mode. This test also checks
  that an appropriate type of lock is passed to
  handler::start_stmt() method.
sql/sql_base.cc:
  Since we no longer set TL_READ as lock type for tables used  
  in simple SELECT right in the parser, in order to preserve  
  behavior for such statements on InnoDB tables when in  
  LOCK TABLES mode with @innodb_table_locks = 0,  
  check_lock_and_start_stmt() had to be changed to convert  
  TL_READ_DEFAULT to an appropriate type of read lock before  
  passing it to handler::start_stmt() method.  
  We do similar thing for TL_WRITE_DEFAULT as this lock type  
  is also supposed to be parser-only type.  
  As consequence read_lock_type_for_table() had to be  
  adjusted to behave properly when it is called from  
  check_lock_and_start_stmt() in prelocked mode.

mysql-test/include/check_no_concurrent_insert.inc

@@ -20,6 +20,9 @@
 --disable_result_log
 --disable_query_log
 
+# Reset DEBUG_SYNC facility for safety.
+set debug_sync= "RESET";
+
 if (`SELECT '$restore_table' <> ''`)
 {
 --eval create table t_backup select * from $restore_table;
@@ -71,5 +74,8 @@ if (`SELECT '$restore_table' <> ''`)
 drop table t_backup;
 }
 
+# Clean-up. Reset DEBUG_SYNC facility after use.
+set debug_sync= "RESET";
+
 --enable_result_log
 --enable_query_log