Better PKCS#11 label creation

Better PKCS#11 label creation

When using the same PKCS#11 URI for a zone that uses different
DNSSEC policies, the PKCS#11 label could collide, i.e. the same
label could be used for different keys. Add the policy name to
the label to make it more unique.

Also, the zone name could contain characters that are interpreted
as special characters when parsing the PKCS#11 URI string. Mangle
the zone name through 'dns_name_tofilenametext()' to make it
PKCS#11 safe.

Move the creation to a separate function for clarity.

Furthermore, add a log message whenever a PKCS#11 object has been
successfully created.