The Addheader patch creates the configuration directive AddHeader.
It adds anything up to newline or '#' to the beginning of mail headers.
You can use a directive in ssmtp.conf like this:

AddHeader=X-security-level: public

I have changed the strtok callss in the read_config to a similarly
looking function, which does not modify the argument string.
I did not, however changed every strtok down the way.