Patch changing how ALTER TABLE implementation handles table locking

and invalidation in the most general case (non-temporary table and
not simple RENAME or ENABLE/DISABLE KEYS or partitioning command).

See comment for sql/sql_table.cc for more information.

These changes are prerequisite for 5.1 version of fix for bug #23667
"CREATE TABLE LIKE is not isolated from alteration by other connections"


mysql-test/include/mix1.inc:
  Extended coverage for behavior of ALTER TABLE statement under LOCK TABLES,
  which should be consistent across all platforms and for all engines.
mysql-test/r/alter_table-big.result:
  Changed test for bug #25044 to use @@debug and injected sleeps
  infrastructure. Extended test coverage for ALTER TABLE's behavior
  under concurrency.
mysql-test/r/alter_table.result:
  Extended coverage for behavior of ALTER TABLE statement under LOCK TABLES,
  which should be consistent across all platforms and for all engines.
mysql-test/r/innodb_mysql.result:
  Extended coverage for behavior of ALTER TABLE statement under LOCK TABLES,
  which should be consistent across all platforms and for all engines.
mysql-test/t/alter_table-big.test:
  Changed test for bug #25044 to use @@debug and injected sleeps
  infrastructure. Extended test coverage for ALTER TABLE's behavior
  under concurrency.
mysql-test/t/alter_table.test:
  Extended coverage for behavior of ALTER TABLE statement under LOCK TABLES,
  which should be consistent across all platforms and for all engines.
sql/mysql_priv.h:
  Made functions reopen_table() and close_handle_and_leave_table_as_lock()
  available outside of sql_base.cc file.
  Changed close_data_tables() in such way that after closing handler
  for the table it leaves TABLE object for it in table cache not as
  placeholder for ordinary name-lock but as placeholder for an exclusive
  name-lock. Renamed this routine to close_data_files_and_morph_locks().
sql/sql_base.cc:
  Made functions reopen_table() and close_handle_and_leave_table_as_lock()
  available outside of sql_base.cc file.
  Changed close_data_tables() in such way that after closing handler
  for the table it leaves TABLE object for it in table cache not as
  placeholder for ordinary name-lock but as placeholder for an exclusive
  name-lock. Renamed this routine to close_data_files_and_morph_locks().
  Also adjusted it so it can work properly not only in LOCK TABLES mode.
sql/sql_table.cc:
  Changed the way in which ALTER TABLE implementation handles table
  locking and invalidation in the most general case (non-temporary table
  and not simple RENAME or ENABLE/DISABLE KEYS or partitioning command)
  
  Now after preparing new version of the table we:
  1) Wait until all other threads close old version of table.
  2) Close instances of table open by this thread and replace them
     with exclusive name-locks.
  3) Rename the old table to a temp name, rename the new one to the
     old name.
  4) If we are under LOCK TABLES and don't do ALTER TABLE ... RENAME
     we reopen new version of table.
  5) Write statement to the binary log.
  6) If we are under LOCK TABLES and do ALTER TABLE ... RENAME we
     remove name-locks from list of open tables and table cache.
  7) If we are not not under LOCK TABLES we rely on close_thread_tables()
     call to remove name-locks from table cache and list of open table. 
  
  Such approach:
  a) Eliminates possibility for concurrent statement to sneak in and get
     access to the new version of the table before ALTER TABLE gets logged
     into binary log.
  b) Ensures that ALTER TABLE behaves under LOCK TABLES in the same way
     on all platforms and for all engines (in 5.0 this was not true) 
  c) Preserves nice invariant that if table is open in some connection
     there is a guarantee that .FRM file for this table exists and is
     properly named.

mysql-test/r/innodb_mysql.result

@@ -796,4 +796,28 @@ lock table t2 write;
 alter table t2 modify i int default 4, rename t1;
 unlock tables;
 drop table t1;
+drop table if exists t1;
+create table t1 (i int);
+insert into t1 values ();
+lock table t1 write;
+alter table t1 modify i int default 1;
+insert into t1 values ();
+select * from t1;
+i
+NULL
+1
+alter table t1 change i c char(10) default "Two";
+insert into t1 values ();
+select * from t1;
+c
+NULL
+1
+Two
+unlock tables;
+select * from t1;
+c
+NULL
+1
+Two
+drop tables t1;
 End of 5.1 tests