core: introduce MANAGER_IS_RELOADING() macro

core: introduce MANAGER_IS_RELOADING() macro

This replaces the old function call manager_is_reloading_or_reexecuting() which
was used only at very few places. Use the new macro wherever we check whether
we are reloading. This should hopefully make things a bit more readable, given
the nature of Manager:n_reloading being a counter.