Do not use deprecated flask.h and av_permissions.h
selinux/flask.h and selinux/av_permissions.h will be completely dropped in the
next SELinux release.
Use string_to_security_class() and string_to_av_perm() to get class and
permission values. The original hardcoded values could be invalid and are
deprecated as the whole flask.h and av_permissions.h header files.
Fixes: https://pagure.io/libuser/issue/44