This pulls a lot of common code out of lxc_user_nic.c.  It also
moves one function from conf.c that was duplicated in lxc_user_nic.c
(It removes a DEBUG statement because (a) it doesn't seem actually
useful and (b) DEBUG doesn't work in network.c).

Also replace the old test of only parsing code with a skeleton for
a full test.  (Note - the test will need some work, it's just there
as do-what-i-mean code example)
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>