shared: add mutable union field NMUtilsNamedValue.{name,value_str}_mutable

NMUtilsNamedValue is a generic tuple that we use for different purposes.
Often we require a mutable string, so add a union alias for that case.

src/libnm-glib-aux/nm-shared-utils.h

@@ -1727,9 +1727,11 @@ typedef struct {
     union {
         NMUtilsNamedEntry named_entry;
         const char *      name;
+        char *            name_mutable;
     };
     union {
         const char *value_str;
+        char *      value_str_mutable;
         gpointer    value_ptr;
     };
 } NMUtilsNamedValue;