Bugfix: Don't read password from stdin for backup --stdin

Restic backup previously tried to read first the password, then the data
to be backed up from standard input. This meant it would often confuse part
of the data for the password.

From now on, restic backup --stdin will exit with the message

    Fatal: cannot read both password and data from stdin

unless the password is passed in some other way (--restic-password-file,
RESTIC_PASSWORD, etc.). To enter the password interactively, a password
command has to be used. For example, on Linux,

    mysqldump somedatabase |
        restic backup --stdin \
            --password-command='sh -c "systemd-ask-password < /dev/tty"'

securely reads the password from the terminal.

https://github.com/restic/restic/issues/2591
https://github.com/restic/restic/pull/4011
