When reading values like access keys or secrets from the terminal, we
would use the `terminal.ReadPassword` function to ensure characters
the user typed were not echo'd back to the console, as a convience.
When standard input was not connected to a tty (which would happen in
some cases like in docker when -t was not passed or in CI), this would
fail with an error about an bad ioctl. Update our logic such that
when standard in is not connected to a terminal, we just read input
normally.
While I was in the area, I unified the code for Windows and *NIX for
these functions.
Fixes#2017
f3fbc1d9ee