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.

src/libostree/libostree-devel.sym

@@ -20,6 +20,10 @@
    - uncomment the include in Makefile-libostree.am
 */
 
+LIBOSTREE_2022.5 {
+global:
+  ostree_kernel_args_append_if_missing;
+} LIBOSTREE_2022.4;
 
 /* Stub section for the stable release *after* this development one; don't
  * edit this other than to update the year.  This is just a copy/paste

src/libostree/ostree-kernel-args.c

@@ -804,3 +804,28 @@ ostree_kernel_args_get_last_value (OstreeKernelArgs *kargs, const char *key)
   const OstreeKernelArgsEntry *e = entries->pdata[entries->len-1];
   return _ostree_kernel_args_entry_get_value (e);
 }
+
+/**
+ * ostree_kernel_args_append_if_missing:
+ * @kargs: a OstreeKernelArgs instance
+ * @arg: key or key/value pair to be added
+ *
+ * Appends @arg which is in the form of key=value pair to the hash table kargs->table
+ * (appends to the value list if key is not in the hash table)
+ * and appends key to kargs->order if it is not in the hash table.
+ *
+ * Since: 2022.5
+ **/
+void
+ostree_kernel_args_append_if_missing (OstreeKernelArgs  *kargs,
+                                      const char *arg)
+{
+  g_autofree char *key = g_strdup (arg);
+  split_keyeq (key);
+ 
+  // Don't insert a duplicate key.
+  if (g_hash_table_contains (kargs->table, key))
+    return;
+ 
+  ostree_kernel_args_append (kargs, arg);
+}

src/libostree/ostree-kernel-args.h

@@ -130,4 +130,8 @@ char **ostree_kernel_args_to_strv (OstreeKernelArgs *kargs);
 _OSTREE_PUBLIC
 char *ostree_kernel_args_to_string (OstreeKernelArgs *kargs);
 
+_OSTREE_PUBLIC
+void ostree_kernel_args_append_if_missing (OstreeKernelArgs *kargs, 
+                                           const char *arg);
+
 G_END_DECLS