allow to select webserver certificate
This feature allow user to select certificate for internal webserver of pfBlocker (lighttpd)
It saves the old /var/unbound/dnsbl_cert.pem file for backward compatibility (Default (auto) cert) and creates a new /var/unbound/dnsbl_web_cert.pem
On each 'update' it compares the fingerprint of cert from the config and cert from this file, and if it's different, updates the dnsbl_web_cert.pem file.
(maybe there is a more elegant way to verify that the certificate is changed)