pass on reboot flag and delete old veth on reboot

pass on reboot flag and delete old veth on reboot

When setting lxc.network.veth.pair to get a fixed interface
name the recreation of it after a reboot caused an EEXIST.
-) The reboot flag is now a three-state value. It's set to
1 to request a reboot, and 2 during a reboot until after
lxc_spawn where it is reset to 0.
-) If the reboot is set (!= 0) within instantiate_veth and
a fixed name is used, the interface is now deleted before
being recreated.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>