iter: rework line iterator API

iter: rework line iterator API

The changes roughly correspond with what we did for chip iterators.

Only perform operations that can fail in gpiod_line_iter_new(), so
that there's no need to check for errors after every call to
gpiod_line_iter_next().

Make gpiod_line_iter an opaque pointer and provide 'new' and 'free'
functions.

Update all users and relevant test cases. Extend the test API with
a new cleanup function.

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