lxc_user_nic: use ppid comparison to decide if we can write to netns

This means that only lxc, as currently designed, can use lxc-user-nic.
It's unfortunate, and I"m open to better ideas.  Unfortunately access(2)
did not work as desired after execing a setuid-root binary.  And the
parent of a userns is not logged anywhere that I know of.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>