gpiosim: add an enum representing line active state

gpiosim: add an enum representing line active state

In order to avoid confusion and stress the fact that "value" represents
the logical state of a GPIO line, add a two-value enum with the names
"ACTIVE" and "INACTIVE" to the libgpiosim API.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>