selinux: make sure we do not try to print missing fields

UID or GID of 0 is valid, so we cannot use that to distinguish whether
calls to sd_bus_creds_get_* succeeded, and the return value from the
function is the only way to know about missing fields. Print "n/a" if
the fields are missing.

CID #1238779
cdrom_id: do not attempt to read past end of buffer

CID #1238437
manager: do not print anything while passwords are being queried

https://bugs.freedesktop.org/show_bug.cgi?id=73942
rpm: add user macros
shell-completion: systemctl set-default,get-default,is-system-running
man: add table of manager states
manager: convert ephemeral to enum

In preparation for subsequent changes.
manager: print warning on console before reboot

It will be printed even if a prompt is blocking other messages.