Set pkcs11 user pin in environment
Set pkcs11 user pin in environment

To hide the user pin from the process list, set it in the environment. That way, only users with access to `cat /proc/$(pidof cfssl)/environ` (typically root) and  can read the user pin. You can store the pin in a file accessible by root only and set the pin as follows:
````USER_PIN=$(cat /etc/cfssl/pin)
     $GOPATH/bin/cfssl ...

If USER_PIN is unset, the behavior is identical to the current one (i.e. you need to provide a pin on the command line).
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