Edge case bug with brightness=0

[epgfm]

mpm 1.12.1

If brightness is set to 0 during power-off on a laptop on battery power, brightness is preserved on next login.

That might be intended behavior, and it's possible to adjust brightness with function keys so desktop becomes visible.

However if one reboots once more after having set a higher brightness using the function keys, session will start again with a black screen. The brightness setting is not remembered.

If one opens the power management, the brightness slider is set to 0. Moving the slider works around the issue for the next logins.

Steps to reproduce:

• Ubuntu MATE 16.04 with MATE 1.12.1 on a laptop that has brightness function keys (not sure if setting it with slider would work too)
• Initiate shutdown, and during the 60 second countdown use brightness key to set brightness to 0
• Boot: black screen after login, use function keys to augment brightness
• Reboot: again black screen after login, last brightness setting disregarded

[epgfm]

This issue might be a duplicate of #187