First steps for bug 14801: add backend support for per-namespace permissions to core...

First steps for bug 14801: add backend support for per-namespace permissions to core. This extends $wgGroupPermissions syntax from $wgGroupPermissions[$group][$right] = bool to $wgGroupPermissions[$group][$right] = array( NS_X => bool ). This is safely backwards compatible; the booleans are still fully supported, and any unset namespace will default to false.

* User::getRights(), User::isAllowed() and User::getGroupPermissions now optionally accept a namespace parameter. If not set, it will check whether the user has the right for all namespaces.
* Anything that uses Title::getUserPermissionsErrorsInternal() automatically supports per-namespace permissions. This includes Title::getUserPermissionsErrors and Title::(quick)UserCan.
* Fix tests that set User::mRights

The next step would be to change all User::isAllowed() to Title::quickUserCan or pass the namespace to User::isAllowed().