core: rename gpiod_version_string() to gpiod_api_version()

core: rename gpiod_version_string() to gpiod_api_version()

Instead of telling the user what type this function returns (they can
check it by looking at the prototype), tell them what the return value
actually represents.

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