Convert reverse proxy settings to new configuration system

Looks good overall to me, one tiny little nitpick though:

+++ b/sites/default/default.settings.phpundefined
@@ -398,35 +398,36 @@ ini_set('session.cookie_lifetime', 2000000);
+ * This setting is required if $conf['system.reverse-proxy']['enabled'] is ¶

Extra space after 'is' should be removed.

Looks good

I don't think this should necessarily be converted to config at all for several reasons:

- whether there's a reverse proxy or not, and what those are, may differ between staging and production. We don't have a concept of environment-specific configuration in the config API yet, or a recommended way to handle this.

- these settings were always meant to be hard-coded in settings.php, because ip_address() is called in drupal_anonymous_user() and watchdog() - so this is adding a config() dependency very, very low down in the system.

- It's also going to add database requests where we previously had none.

Not moving this to CNW, but it needs more discussion. The alternative is to move these globals out of $conf (similar to global $databases).

Also if we do actually convert this to CMI, then it shouldn't be listed in settings.php since that's not going to be the recommended way to set it up any more.

Each request to config is a file system hit when the cache is cold, and a database query when its warm. There are issues open to begin addressing this with static caching and other possibilities. But its probably going to wait until after feature freeze before it goes anywhere.

Managing contextual config differences between dev and prod will most likely be solved by contrib using the new contextual override system (same thing multilingual is using.)

I do agree that this is almost always going to be different between dev and prod, and thus probably isn't best placed in config. I think catch's suggestion of making it a hardcoded variable ala $databases is not a bad idea. Another possibility would be the state system, but that will add the same overhead that config does in terms of db queries (I think.)