After FLUSH STATUS max_used_connections was reset to 0, and haven't
been updated while cached threads were reused, until the moment a new
thread was created.

The first suggested fix from original bug report was implemented:

  a) On flushing the status, set max_used_connections to
  threads_connected, not to 0.

  b) Check if it is necessary to increment max_used_connections when
  taking a thread from the cache as well as when creating new threads


mysql-test/r/status.result:
  Add result for bug#15933.
mysql-test/t/status.test:
  Add test case for bug#15933.
  Fixed typo.
sql/mysql_priv.h:
  Add declaration of refresh_status(), which is now external.
sql/mysqld.cc:
  Remove start_cached_thread() (code moved directly into create_new_thread()).
  Add comment for create_new_thread ().
  In create_new_thread() update max_used_connections when creating new thread
  and when reusing the cached one.
  Move refresh_status() from sql/sql_parse.cc here, on refresh set
  max_used_connections to the current number of connections.
sql/sql_parse.cc:
  refresh_status() moved to sql/mysqld.cc.