Detect bad PHP upload setups

*Title: Detect bad PHP upload setups

Incentive:
Make the setup wizard do some more checks to the PHP
upload system. It should check if the specified upload
directory really exists and is writeable.

*Documentation:
The test will now read out 'upload_tmp_dir' from PHP and check
if that directory exists and can be written to.
If something is wrong it explains why and gives example shell
commands for fixing it.

The setup test 'file_upload' was moved from OptionalTests
to CriticalTests in site.ini.

*Testing:
Reconfigure php.ini and do one of the following:
- Disable php uploading by setting file_uploads to 0
- Change setting 'upload_tmp_dir' to a directory that does not exist.
- Remove all write permission from the directory taken from setting 'upload_tmp_dir'

*SVN Revisions:
8412, 8434, 8435