This is preparation work for context-specific log handlers.

Callers are now encouraged to first initialize the context with
libinput_udev_create_context() and then set the seat for this context with
libinput_udev_assign_seat().

In the upcoming patch to support context-specific log handlers this enables a
caller to set the log handler for a context before any devices are
initialized. Otherwise, a log message generated by a new device may pass a
libinput context that the caller is not yet aware of.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>