If the call to execv() is failed (/usr/bin/unl0kr is absent, for example), the child process will exit with EXIT_FAILURE.
But since the agent does not check the exit code, it will not notice the problem and will return an empty password to systemd.
When the password is used to unlock a PKCS#11 or FIDO2 token, we can waste a limited number of tries or lock the token entirely.
The patch adds a check to avoid this sutuation.
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.
