core: simplify the internal line context structs
core: simplify the internal line context structs

The kernel uapi line handle and event request structures are only
needed for relevant ioctl() calls. After that they can be discarded as
all we need is the file descriptor.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod