Ensure compatibility with MySQL 5.6 which has stricter SQL modes by default.

Disables NO_ZERO_DATE, ONLY_FULL_GROUP_BY, STRICT_TRANS_TABLES, STRICT_ALL_TABLES, TRADITIONAL. Introduces wpdb::set_sql_mode() with an incompatible_sql_modes filter so a plugin can alter the set mode after the fact.

props pento.
fixes #26847.


git-svn-id: https://develop.svn.wordpress.org/trunk@27072 602fd350-edb4-49c9-b593-d223f7449a82