unbound: discard-timeout

[planetf1]

Adds support for 'discard-timeout' to Unbound configuration.

This can be useful when using the option to serve expired responses.

The new option is added just below 'Jostle Timeout' within the advanced panel. I did consider adding it within the serve-expired section, however it's not strictly as tightly related and has more general applicability (as per unbound docs). I also removed once sentence (for help) which made no sense from the unbound docs, and clarified the exact labels used for related settings in the UI.

I tested with a 'live' install to ensure the panel worked, and the value was then placed into /var/unbound/advanced.conf

Based off master

Fixes #7493