Move all mysql_* function calls to wp-db.php

[ComputerGuru]

All calls to database-specific functions should be in a single file for portablility and organization purposes (in this case, wp-db.php for MySQL).

The only mysql_* function not yet constrained to wp-db.php is mysql_get_server_info();

Patch attached. Changes schema.php, upgrade.php, and wp-db.php.

[Nazgul]

As this is a "feature request", I'm bumping the milestone to 2.4.
Feature requests shouldn't be done in maintenance releases.

[westi]

Adding to my review and checkin todo list

[ryan]

The entire function should be moved into wpdb:check_server_version() with it returning an error message via WP_Error.

[westi]

(In [6199]) Move all calls to mysql_ functions to withiWPDB so that t we don't expect any mysql stuff when we are using a custo$wpdb class. Fixes #5127
props ComputerGuru.