MWL#192: non-blocking client API, after-review fixes.

Main change is that non-blocking operation is now an option that must be
explicitly enabled with mysql_option(mysql, MYSQL_OPT_NONBLOCK, ...)
before any non-blocing operation can be used.

Also the CLIENT_REMEMBER_OPTIONS flag is now always enabled and thus
effectively ignored (it was not really useful anyway, and this simplifies
things when non-blocking mysql_real_connect() fails).

mysql-test/r/non_blocking_api.result

@@ -1,3 +1,4 @@
+drop table if exists t1;
 CREATE TABLE t1 (a INT PRIMARY KEY);
 INSERT INTO t1 VALUES (1);
 SELECT * FROM t1;