More helpful error message if PHP_SELF isn't set

More helpful error message if PHP_SELF isn't set

Raw page access was failing in some server configurations with CGI (or
in my case, FastCGI).  This gives a nicer and more correct error
message.  Possibly better solutions:

1) Figure out how to do this without PHP_SELF.

2) Give a warning or error on installation, or even on every page view,
since this will break all raw page viewing.

I ran into this when doing a file copy of my wiki install to another
site, FWIW.  Even with the new error message, I'd have had to look at
the CSS files to figure out what the problem was (the user-visible
symptom was that custom styles didn't work).