add "never" for ttl for passphrase cache

to allow a never-expire for the passphase cache i added a simple "forever"-choice. i don't like this aproach as

 - it doesn't distinguish per key

 - it doesn't distinguish per client

which would be needed. the program should reset the timeout on any usage, not sure if it does already, but if not it should be added. if time allows i will dig into this.