core: add support for bias flags

core: add support for bias flags

Extend the libgpiod API to support the bias flags recently added to the
kernel GPIO uAPI.  The core change is the addition of
GPIOD_LINE_REQUEST_FLAG_BIAS_DISABLE, GPIOD_LINE_REQUEST_FLAG_BIAS_PULL_UP
and GPIOD_LINE_REQUEST_FLAG_BIAS_PULL_DOWN flags to be passed into
line_request functions, and the addition of gpiod_line_bias to return the
bias state of lines.

Variants of the ctxless functions that accept an active_low flag
are added to also accept other flags. The variant names add a "_ext"
suffix to the name of the original function.

Based on initial work by Drew Fustini <drew@pdp7.com>.

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