core: allow to read values of lines configured for events

core: allow to read values of lines configured for events

The kernel allows us to read values of lines on which we're listening
for events, but libgpiod bails out if the user tries to do it.

Modify gpiod_line_get_value_bulk() to select the correct file
descriptor, depending on how the line object is configured and
allow reading values for both normally exported and monitored lines.

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