* API: (bug 17774) API pretends action=query doesn't exist for users without read...

* API: (bug 17774) API pretends action=query doesn't exist for users without read rights
* Instead of hiding read-restricted modules, throw an error when a user without read rights tries to use them
* Do the same for write modules when $wgEnableWriteAPI is false
* Indicate whether a module needs read or write rights in action=help and action=paraminfo
* BREAKING CHANGE: action=purge now requires write rights and, for anonymous users, a POST request