state: fix mod_names_are_active

state: fix mod_names_are_active

This function was always returning -1.
Adding a test, we see that test/state.c treat the is_active functions as
returning booleans, which would treat -1 as success, so we test for > 0
instead (most users would probably get this wrong as well...).
Also update the documentation for the are_active functions, and add a
ATTR_NULL_SENTINEL for gcc __attribute__((sentinel)).

Signed-off-by: Ran Benita <ran234@gmail.com>