Regarding the problem of splitting off the $mode and $ac_* keywords, I've come up with a few solutions,
but the following is my favorite.

    const PROXY_AUTO_DETECT = new proxy_profile("*Auto-Detect*");
    const PROXY_AUTO_CONFIG = new proxy_profile("*Auto-Config*");
    const PROXY_AUTO_SYSTEM = new proxy_profile("*System*");

    keywords("$retry_min", "$retry_max", "$timeout");
    function proxy_auto_config(url) {}

To auto-detect or to use the system proxy, the user just passes the appropriate const profile to
proxy_enable since no arguments are required for these types. To auto-config, call proxy_auto_config()
passing the auto-config url.

This solution works well with the profile hooks as defined thus far, and also with the mode line, which
is expected to use those hooks.