Implement SELinux ls -Z support.

Implement SELinux ls -Z support.

This patch uses lgetfilecon rather than fgetfilecon because
dirtree_parentfd always seems to return -1 in this function. If/when
the SMACK code is fixed to work with dirtree_parentfd, I'll send a
matching patch for SELinux.

In the meantime, this works, and although ls -h is still on my to-do
list, I think this patch is sufficient to let us replace toolbox ls
with toybox ls.