Release version 0.60

• Bump output format version (should have been done for 0.58).
• snap: Add missing staging packages.
• autoconf: Fix AppArmor and SELinux detection
  Bug fix that necessitated a new release: previously, the security
  context displayed with --misc could show as "unknown" due to a
  bug in configure.ac's detection code.
• docs: Add snap details to README.