Remove some of the rubbish that has been accumulating in the default LocalSettings...

Remove some of the rubbish that has been accumulating in the default LocalSettings.php for years, especially code as opposed to configuration data:
* Don't set $IP. The entry point already sets it, so there's no point. Users can still set it if they can figure out a way to do it without breaking things.
* Don't include DefaultSettings.php. Doing that is code, not configuration, and it can easily be done in WebStart/Maintenance. Some non-standard entry points in extensions may be broken by this. That's their fault for being non-standard. Backwards compatibility is preserved thanks to require_once().
* Introduce $wgInvalidateCacheOnLocalSettingsChange, which when set, causes $wgCacheEpoch to be updated in the way that it previously was in the default LocalSettings.php.
* Don't set $wgLocalInterwiki to some nonsense value (possibly including spaces and punctuation in the new installer). It should be only for actual interwiki prefixes. Since most wikis don't have them, I set this to false by default and adjusted the referring code to accept this.
* Removed the guard for web invocation with $wgCommandLineMode set. This ancient code is redundant with modern protections in doMaintenance.php.
* In DefaultSettings.php, fixed fold terminator placement near $wgLoadScript
* Add a web entry point guard. That's one piece of code which really is necessary.