MDEV-12747 - main.mysqld_option_err fails in buildbot with timeout

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.

mysql-test/t/mysqld_option_err.test

@@ -56,4 +56,6 @@ mkdir $MYSQLTEST_VARDIR/tmp/mysqld_option_err;
 --error 2
 --exec $MYSQLD_BOOTSTRAP_CMD --not-known-option --help --verbose >>$MYSQLTEST_VARDIR/tmp/mysqld_option_err/mysqltest.log 2>&1
 
+rmdir $MYSQLTEST_VARDIR/tmp/mysqld_option_err;
+
 --echo Done.

storage/innobase/handler/ha_innodb.cc

@@ -4474,17 +4474,19 @@ innobase_end(handlerton*, ha_panic_function)
 		 	}
 		}
 
-		st_my_thread_var* running = reinterpret_cast<st_my_thread_var*>(
+		st_my_thread_var* r = reinterpret_cast<st_my_thread_var*>(
 			my_atomic_loadptr_explicit(
 			reinterpret_cast<void**>(&srv_running),
 			MY_MEMORY_ORDER_RELAXED));
-		if (!abort_loop && running) {
-			// may be UNINSTALL PLUGIN statement
-			running->abort = 1;
-			mysql_cond_broadcast(running->current_cond);
-		}
-
-		if (!srv_read_only_mode) {
+		if (r) {
+			ut_ad(!srv_read_only_mode);
+			if (!abort_loop) {
+				// may be UNINSTALL PLUGIN statement
+				mysql_mutex_lock(r->current_mutex);
+				r->abort = 1;
+				mysql_cond_broadcast(r->current_cond);
+				mysql_mutex_unlock(r->current_mutex);
+			}
 			pthread_join(thd_destructor_thread, NULL);
 		}