unl0kr: add the CLI option to avoid suffixing a password with a newline character

Appending the newline character '\n' is the right thing to do when you print a password to a tty or a text file.
But different tools process it differently. For example, cryptsetup strips the newline character, but systemd-cryptsetup with a password agent does not.
This commit allows to control the newline character with an option.