tests: support non-MLS enabled SELinux systems

When running "make check" on a Linux system running SELinux with a
non-MLS policy, tests/mkdir/restorecon.sh test fails with:

  chcon: invalid context: root:object_r:tmp_t:s0: Invalid argument

Indeed in such a configuration, contexts cannot have ":s0" suffix.

* init.cfg (get_selinux_type): Refactor this function to here
from various tests.  Update to work with a non-MLS policy.
(mls_enabled_): A new function to detect if MLS is enabled.
(skip_if_mcstransd_is_running_): Update to not skip when
MLS is not enabled.
* tests/mkdir/restorecon.sh: Use a valid non-MLS context when needed.
* tests/install/install-Z-selinux.sh: Likewise.
* tests/cp/cp-a-selinux.sh: Likewise.
* tests/misc/selinux.sh: Likewise.
* tests/misc/chcon.sh: Skip if non-MLS as --range used throughout.
Fixes http://bugs.gnu.org/22631

tests/mkdir/restorecon.sh

@@ -20,11 +20,10 @@
 print_ver_ mkdir mknod mkfifo
 require_selinux_
 
-
-get_selinux_type() { ls -Zd "$1" | sed -n 's/.*:\(.*_t\):.*/\1/p'; }
-
 mkdir subdir || framework_failure_
-chcon 'root:object_r:tmp_t:s0' subdir || framework_failure_
+ctx='root:object_r:tmp_t'
+mls_enabled_ && ctx="$ctx:s0"
+chcon "$ctx" subdir || framework_failure_
 cd subdir
 
 # --- mkdir -Z ---