Reading the password from non-terminal stdin used io.ReadFull with a
byte slice of length 1000.
We are now using a Scanner to read one line of input, independent of its
length.
Additionally, if stdin is not a terminal, the password is read only
once instead of twice (in an effort to detect typos).
Fixes #2203
Signed-off-by: Peter Schultz <peter.schultz@classmarkets.com>
