cannot DEBUG installer

[mikelietz]

On the  debugging wiki page we instruct users to add this line to their config.php:

define( 'DEBUG', true );

If they do so prior to installation, though, the install cannot complete because config gets read once in system/index.php, then again in installhandler.  It dies here at line 88

One workaround for this is to use this line in config.php instead:

if( !defined( 'DEBUG' ) ) define( 'DEBUG', true );

which is overkill the rest of the time, since DEBUG shouldn't ever be already defined before config.php is read.

[michaeltwofish]

We read the config file in the installer so that we don't bother to write it if the values are the same. We could just not check, and write it anyway.