Fixed bug#54486 - assert in my_seek, concurrent

DROP/CREATE SCHEMA, CREATE TABLE, REPAIR.

The cause of assert was concurrent execution of
DROP DATABASE and REPAIR TABLE where first statement
deleted table's file .TMD at the same time as
REPAIR TABLE tried to read file details from the old file
that was just removed.

Additionally was fixed trouble when DROP TABLE try delete
all files belong to table being dropped at the same time
when REPAIR TABLE statement has just deleted .TMD file.

No regression test added because this would require adding a
sync point to mysys/my_redel.c. Since this bug is not present in
5.5+, adding test coverage was considered unnecessary.
The patch has been verified using RQG testing.


sql/sql_db.cc:
  mysql_rm_known_files() modified: ignore possible ENOENT error
  when trying delete all table's files. Such aggressive 
  algorithm permits skip already deleted (in another thread)
  files.
  
  Installation of Drop_table_error_handler as internal error handler
  moved from mysql_rm_db() to mysql_rm_knowns_files() near to place
  where source of possible errors (call to mysql_rm_table_part2) located.
storage/myisam/mi_check.c:
  mi_repair() was modified: set param->retry_repair= 0
  in order to don't call following failover procedure
  in ha_myisam::repair().

storage/myisam/mi_check.c

@@ -1741,6 +1741,8 @@ err:
 			     MYF(MY_REDEL_MAKE_BACKUP): MYF(0))) ||
 	  mi_open_datafile(info,share,name,-1))
 	got_error=1;
+
+      param->retry_repair= 0;
     }
   }
   if (got_error)