thd_destructor_proxy() may miss abort signal if innobase_end() is running
concurrently, which causes server hang in pthread_join() on shutdown.

The problem was that aborting wasn't protected by mutex:
proxy thr: while (!myvar->abort)
end thr: running->abort = 1;
end thr: mysql_cond_broadcast(...);
proxy thr: mysql_cond_wait(...); // nobody to awake it
end thr: pthread_join(...); // waits for proxy thr

Also made main.mysqld_option_err reentrant.