install: with -Z, set default SELinux context for created directories

install: with -Z, set default SELinux context for created directories

* doc/coreutils.texi (install invocation): Update -Z documentation.
* src/install.c (make_ancestor): Set default security context before
calling mkdir() if the -Z option is given.
(process_dir): Call restorecon() on the destination directory if the
-Z option is given.
(usage): Update -Z documentation.
* tests/install/install-Z-selinux.sh: A new test for 'install -Z -D'
and 'install -Z -d' based on tests/mkdir/restorecon.sh.
* tests/local.mk: Reference the test.
* NEWS: Mention the improvement.
Reported at https://bugzilla.redhat.com/1339135
Fixes http://bugs.gnu.org/23868