Merge changes from topic 'file_contexts.bin'

* changes:
  libselinux: support context validation on file_contexts.bin
  libselinux: test for file_contexts.bin format
  libselinux: add selabel_cmp interface and label_file backend
  libselinux: switch to file_contexts.bin
  libselinux: support specifying file_contexts.bin file path
  libselinux: support file_contexts.bin without file_contexts