gets deadlocked when dropping w/ log on"

Log tables rely on concurrent insert machinery to add data.
This means that log tables are always opened and locked by
special (artificial) logger threads. Because of this, the thread
which tries to drop a log table starts to wait for the table
to be unlocked. Which will happen only if the log table is disabled.
Alike situation happens if one tries to alter a log table.
However in addition to the problem above, alter table calls
check_if_locking_is_allowed() routine for the engine. The
routine does not allow alter for the log tables. So, alter
doesn't start waiting forever for logs to be disabled, but 
returns with an error.
Another problem is that not all engines could be used for
the log tables. That's because they need concurrent insert.

In this patch we:
(1) Explicitly disallow to drop/alter a log table if it
    is currently used by the logger.
(2) Update MyISAM to support log tables
(3) Allow to drop log tables/alter log tables if log is
    disabled
At the same time we (4) Disallow to alter log tables to
unsupported engine (after this patch CSV and MyISAM are 
alowed)
Recommit with review fixes.


mysql-test/r/log_tables.result:
  Update result file.
  Note: there are warnings in result file. This is because of CSV
  bug (Bug #21328). They should go away after it is fixed.
mysql-test/t/log_tables.test:
  Add a test for the bug
sql/ha_myisam.cc:
  Add log table handling to myisam: as log tables
  use concurrent insert, they are typically
  locked with TL_CONCURRERENT_INSERT lock. So,
  disallow other threads to attempt locking of
  the log tables in incompatible modes. Because
  otherwise the threads will wait for the tables
  to be unlocked forever.
sql/handler.cc:
  Add a function to check if a table we're going to lock
  is a log table and if the lock mode we want allowed
sql/handler.h:
  Add a new function to check compatibility of the locking
sql/log.cc:
  we shouldn't close the log table if and only
  if this particular table is already closed
sql/log.h:
  add new functions to check if a log is enabled
sql/share/errmsg.txt:
  add new error messages
sql/sql_table.cc:
  DROP and ALTER TABLE should not work on log
  tables if the log tables are enabled
storage/csv/ha_tina.cc:
  move function to check if the locking for the log
  tables allowed to handler class, so that we can
  reuse it in other engines.
storage/myisam/mi_extra.c:
  add new ::extra() flag processing to myisam
storage/myisam/mi_open.c:
  init log table flag
storage/myisam/mi_write.c:
  update status after each write if it's a log table
storage/myisam/myisamdef.h:
  Add new log table flag to myisam share.
  We need it to distinguish between usual
  and log tables, as for the log tables we
  should provide concurrent insert in a
  different way than for usual tables: we
  want new rows to be immediately visible
  to other threads.