Do not check if SELinux is enabled on lowlevel calls to set processlabel

docker exec changes the mount namespace which fools selinux bindings
into thinking SELinux is disabled.  Bindings should just check if
a label is passed in and attempt to use it.  Docker will not call these
functions with a label if SELinux is disabled.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge pull request #222 from rhatdan/exec

Do not check if SELinux is enabled on lowlevel calls to set processlabel