For those who need it, the script asks for how many days the new client
should be valid. This defaults to 3650 days. Then it sets the
```EASYRSA_CERT_EXPIRE``` variable accordingly.
This script is meant to be simple which means it is for those who are
not tech-savvy to handle the complex task of installing and configuring
OpenVPN.
However if the user has a large number of clients and all of them are
valid for 10 years, it is very hard to keep track of them. This PR helps
them to set a reasonable validity period, while the default is the same.
---------
Co-authored-by: Stanislas Lange <git@slange.me>
