handle: use dynamic major/minor allocation (ABI break)

handle: use dynamic major/minor allocation (ABI break)

Instead of requiring 1 major per domain, we now allocate major/minor
combinations dynamically. So far, only a single major is allocated during
module init, but the code can easily be extended to even make those
dynamic. However, device-cgroups require us to have a fixed major. User
space must be aware that major/minor numbers no longer have any specific
meaning. Each major/minor combination might be assigned to any domain
and/or endpoint! Apart from this semantics change, the ABI stays the same.

Furthermore, this patch reworks the kdbus_domain_new() and kdbus_ep_new()
functions to avoid races against UEVENT_ADD. Both objects must be active
before we call device_add() and thus produce UEVENT_ADD.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>