This adjusts `command.PromptForConfirmation` in `cli/command/utils.go` to use `bufio`'s `ReadLine` rather than using `fmt.Fscan` for reading input, which makes `<Enter>` properly accept the default value of "No" as one would expect.
This new code actually came from `cli/command/plugin/install.go`'s `acceptPrivileges` function, which I've also refactored here to use `command.PromptForConfirmation` as it should.
Additionally, this updates `cli/command/plugin/upgrade.go`'s `runUpgrade` function to use `command.PromptForConfirmation` for further consistency.
Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>