Previously, it only pretended to rewrite the configuration file, so it
set or cleared OPT_MUST_SAVE but never changed or output any options.
Now, it actually sets the options when ELinks is loading the
configuration file. Also, when ELinks is rewriting the configuration
file, it now compares the values in the included file to the current
values of the options, and sets or clears OPT_MUST_SAVE accordingly.
Introduce get_option_shadow. This routine takes an option, the tree under which that option resides, and another tree. It returns a corresponding option with parallel ancestry in the second tree, creating that option and ancestry if it does not already exist.
Add enum copy_option_flags.
Add the CO_NO_LISTBOX_ITEM flag, which is used to suppress listbox creation for shadowed trees.
Add the CO_SHALLOW flag, which is used to suppress the duplication of unwanted children for shadowed trees.
Add a flags parameter to copy_option and tree_dup (and out of necessity, struct option_type_info and str_dup).
