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.