setpriv: Fix --selinux-label

This commit:

    commit 05cef8eafbf169a8da1d2831f5ed3d5b1b81aaad
    Author: Sami Kerola <kerolasa@iki.fi>
    Date:   Sat Apr 13 20:54:59 2013 +0100

        setpriv: check writing to a file descriptor was successful

broke --selinux-label.  It checks whether fsyncing /proc/self/attr/exec
succeeds, and it doesn't, because fsync isn't supported on that file.

I have stealthily improved the error message as part of this fix.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>