The last checkin broke the wiki on installations where TMP is not set

The last checkin broke the wiki on installations where TMP is not set
and the upload directory is not writable (ie, default install!)

Now checks a system-dependent default temp dir first (/tmp or
C:\Windows\Temp) and then falls back to upload dir. (TMP takes
priority over either.) If the dir is not writable, print out
an explanatory message for the poor sap trying to figure out
why the wiki isn't working.

Some of this crud shouldn't go in PHPTAL if possible, since it's
rather mediawiki-specific.