core: provide functions for reading multiple line events at once

core: provide functions for reading multiple line events at once

The kernel allows us to read multiple (up to 16) line events with
a single read() call, but up to this point libgpiod only supported
reading one event at a time. Add two new functions that allow users
to read more events at the same time.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>