Prevent Docker daemon from starting with btrfs graph driver if SELinux is enabled.

The Docker btrfs graph driver does not interact well with SELinux at present.
If btrfs mounts the same file in several locations, the same SELinux label will
be applied to all mountpoints. In the context of the graph driver, things such
as shared libraries become inaccessible to containers due to SELInux, causing
all dynamically linked applications to fail when run in a container.

Consequently,  error when we detect the daemon is being run with SELinux enabled
and the btrfs driver.

Docker-DCO-1.1-Signed-off-by: Matthew Heon <mheon@redhat.com> (github: mheon)
Add note in CLI documentation to indicate incompatability between BTRFS driver and --selinux-enable

As this configuration now produces an error message (and does not work regardless), warn users
in CLI documentation that the --selinux-enabled=true flag conflicts with the BTRFS storage driver.

Docker-DCO-1.1-Signed-off-by: Matthew Heon <mheon@redhat.com> (github: mheon)
Note in Docker manpage that --selinux-enabled flag cannot be used with BTRFS driver

Docker-DCO-1.1-Signed-off-by: Matthew Heon <mheon@redhat.com> (github: mheon)