iter: rework chip iterators

iter: rework chip iterators

We can significantly simplify the chip iterators by performing all
actions that can yield an error in gpiod_chip_iter_new(). Instead of
opening each subsequent chip in gpiod_chip_iter_next(), open them all
in the new function and make next just return an active handle.

Fix relevant test cases and macros, remove related routines that are
no longer needed.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>