run/uid0: tint the terminal background color (and add new --background= switch)

run/uid0: tint the terminal background color (and add new --background= switch)

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.