Add "ostree admin instutil", move selinux-ensure-labeled there

There are going to be a few utilities that are only useful for
installers and disk image creation tools.  Let's not expose them all
at the toplevel; instead, hide them under "instutil".
libostree: Add ostree_bootconfig_parser_clone()

This will be necessary to fix ostree_deployment_clone(), but is
potentially useful on its own for other consumers.
libostree: Fix ostree_deployment_clone() to also clone bootconfig

This way one can easily mutate it for a new deployment list.
libostree: Add ostree_sysroot_deployment_set_kargs()

It turns out people sometimes want to be able to change the kernel
arguments.  Add a convenient API to do so for the current deployment.

This will be used by Anaconda.
core: Add "admin instutil set-kargs"

This will be used by Anaconda as a convenience command to set the
bootloader arguments.
pull: End status line on error as well

This way we don't append the error to the current line.