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.
Merge pull request #3015 from jpoimboe/dockerinit-libvirt-prereqs

dockerinit: drop capabilities