Update examples/loopback.pl

Added ENOTDIR to POSIX exports (was missing).
Update examples/loopback.pl

Lchown might not be available on the platform, or it might just not work out-of-the-box, as for me today ("Can't locate loadable object for module Lchown"). Fallback gracefully in these cases.
Update examples/loopback.pl

I might misunderstand usage, but I think, without a path in @ARGV, the script should spit out some usage info, so a user doesn't have to dig into the code for a quick test-drive.
Update examples/loopback.pl

Unix::Mknod might not be available or might not work - same as for Lchown. Graceful fallback. Is "POSIX::errno();" the right error code?
Update examples/readdir.pl

$file already sanitized in filename_fixup, right?
Merge pull request #13 from isync/master

Tweaks on loopback.pl example