This adds a new --background= switch that allows specifiying a
background color for the terminal while the tool runs.
It also teaches the tool when invoked as uid0 to tint the terminal in a
reddish hue when operating as root, and in a yellowish hue when
operating as any other user.
This should highlight nicely when the user is operating with elevated
privileges, or changed privileges.
This turns "systemd-run" into a multi-call binary. When invoked under
the name "uid0", then it behaves a bit more like traditional "sudo".
This mostly means defaults appropriuate for that, for example a PAM
stack, interactivity and more.
Fixes: #29199