logind: rename manager_item_for_handle() → handle_action_lookup()

logind: rename manager_item_for_handle() → handle_action_lookup()

The function has nothing to do with any Manager object, hence drop that
from the name. And it actually looks something up by handle *action* not
by *handle*, hence the old name was a bit misnomer. Let's call it
handle_action_lookup(), as it queries handle action metainfo for a
handle action.

Also, let's make sure it behaves more like our usual functions that
lookup some fixed data from some enum value/int: let's return NULL if we
don't find it.