priv: create chroot if it does not exist

This is not the ideal situation since the chroot should maybe contain
some additional stuff (like `/etc/localtime` with GNU libc) to make it
work properly, but this is more convenient that leaving the user does
exactly the same things. Packagers are still expected to populate the
chroot correctly before starting lldpd.
priv: make inability to set write filter as fatal

On BSD, if the userland supports an ioctl, the kernel is in sync and
also supports the ioctl. So, if BIOCSETWF exists, it should succeed.
priv: add ability to lock filter on Linux

This is a new feature which should be available in Linux 3.9. See:
 http://git.kernel.org/?p=linux/kernel/git/davem/net-next.git;a=commit;h=d59577b6ffd313d0ab3be39cb1ab47e29bdc9182