(dev/core#217) PrevNext - Sanitize the `markSelection()` contract

(dev/core#217) PrevNext - Sanitize the `markSelection()` contract

The contract feels quirky -- e.g.

* Who would guess that `markSelection()` defaults to `$action == 'unselect'`?
* What's the point of accepting `$entity_table` if it's never used?

Fortunately, this function is only called from `CRM_Contact_Page_AJAX::selectUnselectContacts`,
so it's fairly easy to audit and see that:

* The `$action` is always passed in -- it never relies on the default value.
* The `$entity_table` is never specified explicitly -- it always relies on the default value.