New optional data collection for SELinux plug-in

The SELinux plug-in can now optionally collect object lists from
the semanage command. Since this command loads the SELinux python
run time (~4s/command) it can add considerably to the overall run
time of sosreport - for this reason these commands are disabled by
default and may be enabled via the selinux.list option:

 # sosreport -k selinux.list

With the option turned on we also collect:

  semanage fcontext -l
  semanage login -l
  semanage port -l
  semanage user -l

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>