Confine I2P with AppArmor.

Bugfix on 2.0~beta1.

Will-fix: #10925

config/chroot_local-includes/lib/systemd/system/i2p.service.d/AppArmor.conf

+[Service]
+# XXX:Stretch: on Jessie, AppArmorProfile=system_i2p is a no-op, since
+# AppArmor support was enabled in Debian's systemd 218-4 (#10925).
+ExecStart=
+ExecStart=/usr/sbin/aa-exec --profile=system_i2p -- /usr/sbin/wrapper "$I2P_ARGS"