Debian: Merge back release packaging into development

Signed-off-by: Markus Frosch <markus@lazyfrosch.de>

Refs #5816
Debian: update changelog to warn about package base
Plugins: add fping4/6 commands

refs #6225
Documentation: fix ssh example
Notifications: Check if disabled when sending reminder

refs #6510
Fix: Icinga doesn't send SetLogPosition messages when one of the endpoints fails to connect

fixes #6537
Add missing license headers

refs #6493