secmark: export secctx, drop secmark in procfs

The current secmark code exports a secmark= field which just indicates if
there is special labeling on a packet or not.  We drop this field as it
isn't particularly useful and instead export a new field secctx= which is
the actual human readable text label.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: James Morris <jmorris@namei.org>