logind: let's change the type of the runtime directory size to uint64_t

logind: let's change the type of the runtime directory size to uint64_t

Externally it's an uint64_t anyway, and internally we most just
initialize it to physical_memory() which returns uint64_t, hence there's
exactly zero value in using it as size_t internally. Hence, let's fix
that, and use uint64_t everywhere.