_nm_register_setting() and _nm_register_setting_impl() are called from within
the GType constructor for the NMSetting subtype. As such, at that point it
runs inside a g_once_init_enter() block. However, each implementation
for initializing the GType has a separate g_once_init_enter() variable, hence,
if two threads create GType instances for different NMSetting subclasses, there
is a race.

libnm is not thread safe. However, it should be at least thread safe
with respect to constructing the GType instances.