daemon: Add '--substitute-urls' option.
* nix/nix-daemon/guix-daemon.cc (GUIX_OPT_SUBSTITUTE_URLS): New macro.
(GUIX_OPT_NO_BUILD_HOOK, GUIX_OPT_GC_KEEP_OUTPUTS,
GUIX_OPT_GC_KEEP_DERIVATIONS): Renumber.
(options): Add '--substitute-urls'.
(parse_opt): Honor it.
(main): Add 'settings.set' call for the default "substitute-urls"
value.
* guix/scripts/substitute-binary.scm (daemon-options,
find-daemon-option): New procedures.
(%cache-url): Define based on the "substitute-urls" daemon option.
* doc/guix.texi (Invoking guix-daemon): Document '--substitute-urls'.
(Substitutes): Mention it.