Note that this is the infrastructure, but all relevant get_opt_* calls must be changed to pass the session so that the domain-specific options are looked up.
Add @want_domain parameter to parse_set_common and read in the domain-name if the flag is set.
Add parse_set_domain wrapper for parse_set_common.
Add "set_domain" configuration directive with the following syntax: set_domain domain option = value
Modify create_config_string and smart_config_output_fn to spit out domain-specific option trees.
Add @smart_config_output_fn_domain global variable to facilitate this.
Define structure domain_tree and define list @domain_trees.
Add routine get_domain_tree to find or, if necessary, create the shadow tree for the given domain name.
Add routine get_domain_option search for an option in all domain shadow-trees and return the option in the best matching domain tree.
Modify get_opt_ to use get_domain_option to check for domain-specific options.
Add clean-up routine done_domain_trees, called on exit, to free any domain trees.
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).
