Check list of backends in the usr.sbin.cups AppArmor profile (2018 edition)

Originally created by @intrigeri on #15029 (Redmine)

1. Check that what follows is still needed: see “[apparmor] cupsd profile: list of backends”, August 2015

2. Check that the list of backends we ship in /usr/lib/cups/backend are all listed in the (patched) /etc/apparmor.d/usr.sbin.cups:

• backends shipped in the cups-daemon package should have ixr
• other backends should have Cx -> third_party

3. Create a ticket to do the same next year.

Feature Branch: bugfix/15029-AppArmor-cups-backends

Related issues

• Related to #9963 (closed)
• Related to #15030 (closed)
• Blocks #13245 (closed)
• Blocked by #15132 (closed)
• Blocked by #15270 (closed)