are pending
This is to support pending deployments instead of rasing assertion.
For example:
```
$ sudo rpm-ostree kargs --append=foo=bar
$ sudo ostree admin kargs edit-in-place --append-if-missing=foobar
```
After reboot we get both `foo=bar foobar`.
Fix https://github.com/ostreedev/ostree/issues/2679
update all existing deployments in place
Example:
$ sudo ostree admin kargs edit-in-place --append-if-missing=rw
See https://github.com/ostreedev/ostree/issues/2617
This will not add duplicate key, if there is `TESTARG=VAL1` in the
kernel arguments, `--append-if-missing=TESTARG=VAL2` will be ignored.