Allow installing and running the wiki on a server where ini_set has been

Allow installing and running the wiki on a server where ini_set has been
disabled via disable_functions in php.ini (bug #928168)

The PHPTal skin doesn't function unless the PEAR bits are in include_path,
so it's disabled if ini_set can't be used. If the files are manually placed
or a real PEAR installation is available with the right stuff, one can
use $wgUsePHPTal = true;

There are likely other things that need to be taken into account for
restricted environments. Maintenance scripts haven't been tweaked for
such an environment yet either.