examples/unsubscribe.milter: support unique mailto:
Instead of providing a generic "mailto:foo+unsubscribe@example.com"
address in List-Unsubscribe which requires confirmation, replace it
with a mailto: header with a unique subject which contains the same
unique ID we put in the https:// URL.
This makes it easier for some MUAs without https:// support to
unsubscribe with a single action via the List-Unsubscribe header.