The problem was a "self-deadlock" if the connection issuing INSERT DELAYED
had both the global read lock (FLUSH TABLES WITH READ LOCK) and LOCK TABLES
mode active. The table being inserted into had to be different from the 
table(s) locked by LOCK TABLES.

For INSERT DELAYED, the connection thread waits until the handler thread has
opened and locked its table before returning. But since the global read lock
was active, the handler thread would be unable to lock and would wait for the
global read lock to go away.

So the handler thread would be waiting for the connection thread to release
the global read lock while the connection thread was waiting for the handler
thread to lock the table. This gave a "self-deadlock" (same connection,
different threads).

The deadlock would only happen if we also had LOCK TABLES mode since the
INSERT otherwise will try to get protection against global read lock before
starting the handler thread. It will then notice that the global read lock
is owned by the same connection and report ER_CANT_UPDATE_WITH_READLOCK.

This patch removes the deadlock by reporting ER_CANT_UPDATE_WITH_READLOCK
also if we are inside LOCK TABLES mode.

Test case added to delayed.test.