selinux: fix --context=CTX for cp and diagnose defaultcon() errors

selinux: fix --context=CTX for cp and diagnose defaultcon() errors

* src/selinux.h (ignorable_ctx_err): A new function used
to determine if a warning should be given after a call
to defaultcon() or restorecon().
* src/cp.c (main): Fix the setfscreatecon() call to use
the argument passed by the user.
* src/mkdir.c (make_ancestor): Show all but "ignoreable" errors
from defaultcon() and restorecon().
* tests/misc/selinux.sh: Add a test run as root in selinux enforcing
mode, to ensure cp --context=invalid is honored and fails immediately.