Add configuration option for no input device.

Add configuration option for no input device.

As it has been discussed in the past [1], running Weston
without any input device at launch might be beneficial for
some use cases.

Certainly, it's best for the vast majority of users (and
the project) to require an input device to be present, as
to avoid frustration and hassle, but for those brave souls
that so prefer, this patch lets them run without any input
device at all.

This introduces a simple configuration in weston.ini:
  [core]
  require-input=true

True is the default, so no behavioral change is introduced.

[1] https://lists.freedesktop.org/archives/wayland-devel/2015-November/025193.html

Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>