Add new PDO mysql connection attr to control multi statements option


mysqli does not set the CLIENT_MULTI_STATEMENTS flag on connect, so a query with multiple statements fails.

For PDO mysql, it hard-codes the CLIENT_MULTI_STATEMENTS flag in the connection, so there is no way to disable it.  However, if using native prepares, sending multiple statements fails on the server.  This is inconsistent.

This PR adds a new MySQL-specific attribute that can be only set at connection time to explicitly enable or disable multi statements.  As written, it leaves the default the same as current master (enabled).

While this PR is against master, I think this change falls somewhere between a feature and a bugfix and should be back-ported to all supported versions.

The motivation for this is the severity of the recent SQL injection vulnerability in Drupal.  If we had any way to disable multi statement in PDO (which is used in Drupal 7.x but not 6.x), we would have, and the vulnerability would have been significantly mitigated. see: https://www.drupal.org/SA-CORE-2014-005

Ok, so does it make sense to guard the class constant using
```
#ifdef CLIENT_MULTI_STATEMENTS
```

or since it would be ignored if somehow that wan't defined, it's better to always define it to avoid PHP code having to check if it exists?