logind: fix session-device dbus notify

Had this fix lying around here for some time. Thanks to missing
type-checking for va-args we passed in the actual major/minor values
instead of pointers to it. Fix it by saving the values on the stack first
and passing in the pointers.
logind: check whether first drmSetMaster succeeded

The initial drmSetMaster may fail if there is an active master already. We
must not assume that all existing clients comply to logind rules. We check
for this during session-activation already but didn't during device setup.
Fix this by checking the return code.

As drmSetMaster has had horrible return codes in the past (0 for failure?
EINVAL for denied access, ..) we need to be quite pedantic. To guarantee
an open file-descriptor we need to close the device and reopen it without
master if setting master failed first.
logind: send PropertyChanged during deactivation

We only send the PropertyChanged signal for the to-be-activated session
but not for the to-be-deactivated one. Fix that so both listeners get
notified about the new state.
logind: run with CAP_SYS_ADMIN

DRM Master access requires CAP_SYS_ADMIN, yay! Add it to the capability
bounding set for systemd-logind. As CAP_SYS_ADMIN actually allows a huge
set of actions, this mostly renders the restriction-set useless. Anyway,
patches are already pending to reduce the restriction on the kernel side.
But these won't really make it into any stable-release so for now we're
stuck with CAP_SYS_ADMIN.
logind: remove fbdev session-device support

fbdev does not support access-handover so it is quite useless to route it
through logind. If compositors want to use it they ought to open it
themselves. It's highly recommended to be ignored entirely, though. fbdev
is about to be deprecated in the kernel.