• Introduce new GEOM utility function g_is_geom_thread(), returning true

if specified thread is one of GEOM threads (g_up/g_down/g_event).

• Use this function instead of td->td_no_sleeping != 0 (that was not

supposed to be public KPI) to detect whether we can sleep in this context.

From this moment assume that G_CF_DIRECT_SEND consumer flag means not only
reenterability, but also that caller context is sleepable (if we are not
already in one of GEOM threads where sleep artificially denied by design).