Add a Weston cmd line argument (force advertise pointer) to allow clients started by Weston get pointer related events

I don't know if we have the means currently get information, from the front-end, the libweston/input and check for arguments. Other this should rather straight forward as evdev_device_create() should advertise a pointer, as it was removed in e825fe38.

This has been requested from !1471 (merged)