core: improve error handling

core: improve error handling

Rework the way we verify that line bulk objects are consistent: split
the verify_line_bulk() function into sub-routines, make the names more
self-explanatory and add other minor fixes & tweaks.

Improve the consistency of errno codes: set errno to EPERM if the line
for which gpiod_line_event_get_fd() is called is not configured for
event monitoring.

We were already testing that line requests fail if we pass them a line
bulk containing lines from different chips, but add an analogous test
for gpiod_line_get_value_bulk().

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