tools: add gpionotify

tools: add gpionotify

Add a gpionotify tool, based on gpiomon, to report line info change
events read from chip file descriptors.

Inspired by the gpio-watch tool in the linux kernel, but with gpiomon
features such as custom formatted output, filtering events of
interest and exiting after a number of events, so more useful for
scripting.

Default output is minimalist, so just time, event type and line id.
Full event details are available using the custom formatted output.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
[Bartosz: coding style tweaks]
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>