core: add support for SET_CONFIG

core: add support for SET_CONFIG

Extend the libgpiod API to support the setting line configuration using the
GPIO GPIOHANDLE_SET_CONFIG_IOCTL uAPI ioctl.

The core change is the addition of gpiod_line_set_config, which provides a
low level wrapper around the ioctl.

Additionally, higher level helper functions, gpiod_line_set_flags,
gpiod_line_set_direction_input, and gpiod_line_set_direction_output provide
slightly simplified APIs for common use cases.

Bulk forms of all functions are also provided.

Documented the fields of gpiod_line to better identify the purpose of
each where the field name alone is not sufficiently clear, and to
indicate which flags are applicable to each field.

Implementation includes a few helper functions that serve to keep the
code tidier and are consistent with similar helper functions already
present.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>