RFE: Add a hidden option to ostree admin kargs edit-in-place to

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.

apidoc/ostree-sections.txt

@@ -727,6 +727,7 @@ ostree_kernel_args_replace_argv
 ostree_kernel_args_append
 ostree_kernel_args_append_argv
 ostree_kernel_args_append_argv_filtered
+ostree_kernel_args_append_if_missing
 ostree_kernel_args_new_replace
 ostree_kernel_args_delete
 ostree_kernel_args_delete_key_entry