Previously nm_ppp_manager_stop() would return a handle which
makes it easy to cancel the operation.

However, sometimes, we may want to cancel an operation based on
an GCancellable. So, extend nm_ppp_manager_stop() to hook it
with a cancellable.

Essentially, move the code from nm-modem.c to nm-ppp-manager-call.c,
where it belongs and where the functionality gets available to every
component.