Restore "stty echo" upon receiving SIGINT when asking for password

Roman Lackorequested to merge
xlacko1/fi-printers:interrupt into master

Shells do not generally have to restore line settings when a subprocess exists. This means that when the program asks for a password and is interrupted by Ctrl-C, stty -echo may remain in effect in these types of shells.

This MR restores stty echo when password prompt is interrupted by Ctrl-C.

(It also removes code duplication by introducing prompt_secret() subroutine.)

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