The `backlight` file seems to not receive modification events for
inotify (it does receive other events though).
These other events still trigger an update, but the value read is still
the old value because the events arrive just slightly before the file is
updated.
The new `poll-interval` setting will inject an event every X seconds to
force an update.
If the actual_brightness file is used, the interval is set to 0 and thus
turned off.
This does not add any more wakeups to the module, the inotify modules
wake up every second anyway and during that time, the interval is
checked.
Fixes#2835
aebdb5328a