Add sysfs probe for locating backlight brightness controls

After the changes brightness controls will use first match from
1. File/dir configuration from ini-files
2. Legacy hardcoded paths logic
3. Scanning /sys/class/backlight/ directory
Do not allow out of hw bounds brightness values

Fixes the target hw brightness value calculation at the
module init function (percentage values were used as hw
values which can lead to black screen if maximum hw
value is low enough).

As a protection against similar problems elsewhere, all
brightness value updates go through write_brightness_value()
function that clips values to a valid hw range.

Also added probe for /sys/class/leds/lcd-backlight that
seems to be used on some Android kernels instead of the
/sys/class/backlight/xxx entries.
Reset existing value array when reading from persistent storage file

Oops, arrays got longer on every mce restart...
Use mce_log() levels, not syslog() priorities

While LL_ERR == LOG_ERR etc, we should use LL_XXX with mce_log()
Merge pull request #28 from spiiroin/brightness_probe

Try to discover display brightness controls automatically from /sys/class/backlight