dockerinit: put args in a struct
dockerinit: refactor error handling
dockerinit: drop capabilities

Drop capabilities in dockerinit instead of with lxc utils, since
libvirt-lxc doesn't support it.

This will also be needed for machine container mode, since dockerinit
needs CAP_SYS_ADMIN to setup /dev/console correctly.
dockerinit: set hostname

Set the hostname in dockerinit instead of with lxc utils.  libvirt-lxc
doesn't have a way to do this, so do it in a common place.
dockerinit: set IP address

Set the IP address in dockerinit instead of lxc utils, to prepare for
using libvirt-lxc.
dockerinit: propagate "container" env variable from lxc

Lxc (and libvirt) already set the "container" env variable
appropriately[1], so just use that.

[1] http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/