Fix issue when packing netmails for multiple different linked-nodes

Introduced in commit 0e38cb0f (9 months ago), the first linked-node config
(including packet password!) for the first packed netmail message would be
reused for subsequent packed netmail messages.

Thank you to Wilfred van Velzen (2:280/464) for reporting this problem!

Additional debug-level log message when using a packet password for a newly
created packet.