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)
When I was trying to figure out why the SELinux labels were not being set, I found this bug,  Basically the label is set per Thread, not for the process.  So this change makes Getexeccon match Setexeccon.

Currently we never use this call except for debugging problems, so no risk of this breaking anything in docker.