glib-compat: sync local definition of g_clear_pointer() with upstream glib and remove atomic operations

Upstream glib changed g_clear_pointer() not to use atomic functions.
Update or local definition to b1dd594a22e3499caafdeccd7fa223a032b9e177
glib/gmem.h (glib 2.41.3).

(fixup whitespace to match our style).

See also the related bug https://bugzilla.gnome.org/show_bug.cgi?id=733969
from glib.

Signed-off-by: Thomas Haller <thaller@redhat.com>
clients: only handle secret requests for connection being explicitly activated

When a connection is being activated, nmcli could ask for secrets for another
connection, which might confuse users. We check the request now and only ask
for secrets of connection being activated.

Test case:
$ nmcli con up my-ethernet0
Passwords or encryption keys are required to access the wireless network 'Red Hat'.
Warning: password for '802-1x.identity' not given in 'passwd-file' and nmcli cannot ask without '--ask' option.