core: provide gpiod_line_config_set_output_values()

core: provide gpiod_line_config_set_output_values()

Currently if user wants to use the same settings for a set of requested
lines with the exception of the output value - they need to go through
hoops by updating the line settings object and adding it one by one to
the line config. Provide a helper function that allows to set a global
list of output values that override the settings. For details on the
interface: see documentation in this commit.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>