Require Boost.Iostreams

See merge request !11

Use GitHub tags page in debian/watch

See merge request !10

This allows building packages after the tag was pushed but before the corresponding GitHub release was published.

The same change was already done in deb-icinga2:
deb-icinga2@e8e9e552

Add bullseye

See merge request !7

[ci skip]

This will ease dependency installation on Raspbian, where aptitude can
not be used (pbuilder-satisfydepends-aptitude).

Make stretch the default version

[ci skip]

chown several files and directories to the daemon user.