API: use gpiod_ctxless_ as prefix for the high-level API

API: use gpiod_ctxless_ as prefix for the high-level API

As suggested by Linus Walleij: the word 'simple' is generally
subjective and the high-level routines provided by libgpiod are not
necessarily simple, but rather contextless, as they don't require any
resource managemend. Use 'gpiod_ctxless_' as prefix for all high-level
symbols.

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