This adds selinux_mls_enabled() and selinux_enabled() to detect a)
whether selinux is MLS aware (ie supports selevel) and b) whether
selinux is enabled.  If selinux is not enabled, all selinux operations
are punted on -- same as if python's selinux module were not available.
In set_context_if_different(), I now iterate over the current context
instead of the context argument.  Even if the system supports MLS, it
may not return the selevel from selinux.lgetfilecon().  Lastly, this
drops selinux_has_selevel() in lieu of the current approach.