libnm: deprecate nm_client_check_connectivity() in 1.22

The previous commit marks all synchronous libnm API as deprecated.
In practice, the macro _NM_DEPRECATED_SYNC_METHOD expands to
nothing, because there is no immediate urgency to force users
to migrate.

However nm_client_check_connectivity() is especially bad: it
makes a synchronous call and then updates the content of the
cache artificially. Usually, NMClient's cache of D-Bus objects
is only updated by "PropertiesChanged" D-Bus signals.
nm_client_check_connectivity() instead will act on the response to
the "CheckConnectivity" D-Bus call -- a response that is picked
out of order from the ordered sequence of messages --  and will
update the cache instead of honoring the usual "PropertiesChanged"
signal.

I think such behavior is fundamentally broken. For a trivial property like
NM_CLIENT_CONNECTIVITY such behavior is odd at best. Note how applying
this approach to other functions (like nm_client_deactivate_connection(),
which would affect a much larger state) would not be feasible.

I also imagine it to be complicate to preserve this behavior when
reworking libnm, as I plan to do.

See also commit b799de281b ('libnm: update property in the manager
after connectivity check'), which introduced this behavior to "fix"
bgo#784629.

libnm/nm-client.h

@@ -292,6 +292,7 @@ NMClientPermissionResult nm_client_get_permission_result (NMClient *client,
 NMConnectivityState nm_client_get_connectivity          (NMClient *client);
 
 _NM_DEPRECATED_SYNC_METHOD
+NM_DEPRECATED_IN_1_22
 NMConnectivityState nm_client_check_connectivity        (NMClient *client,
                                                          GCancellable *cancellable,
                                                          GError **error);