The GetAuthenticationModes function modifies the receiver, so its
function signature should use a *exampleBroker to indicate that it does
modify the receiver.

The broker can handle multiple requests at once, even for the same
username, so we need some write safety when doing changes on the broker
such as adding/deleting keys from the currentSessions map

We were using static strings for handling the broker responses and this
was prone to error and would complicate future changes, so those values
should be well-defined somewhere to avoid those obstacles

This adds both AbortSession and CancelIsAuthorized to the Brokerer
interface. Those methods allow us to handle cancellation of the calls
through communication with the actual broker, instead of solely relying
on context cancellation as it is not possible to do it through dbus.

This is an operation that we do constantly, so it deserves its own
function.

Now that we have a proper cancellation API, this updates the pam
module with the new way of interacting with those calls.

AbortSession is now an exported method that can be used through the
client