systemd-sleep: Set SYSTEMD_SLEEP_ACTION for systemd-sleep hooks.
When suspend-then-hibernate is called, hooks have no ability to determine which
stage of the request is being handled; they only see 'pre' and 'post' with the
verb 'suspend-then-hibernate'. This change introduces an environment variable
called SYSTEMD_SLEEP_ACTION that contains the name of the action that is
processing: 'suspend', 'hibernate', 'hybrid-sleep', or
'suspend-after-failed-hibernate'.