capability-util: add CAP_MASK_ALL + CAP_MASK_UNSET macros

capability-util: add CAP_MASK_ALL + CAP_MASK_UNSET macros

We should be more careful with distinguishing the cases "all bits set in
caps mask" from "cap mask invalid". We so far mostly used UINT64_MAX for
both, which is not correct though (as it would mean
AmbientCapabilities=~0 followed by AmbientCapabilities=0) would result
in capability 63 to be set (which we don't really allow, since that
means unset).