Don't parse disable_functions INI value

In wfDebugBacktrace() and wfShellExecDisabled(), use function_exists()
instead of getting the value of the disable_functions INI setting,
then splitting it into a list of functions, then checking that list
for the function to be called.

Also removed the check for Zend Optimizer, an old PHP extension that
does not work with PHP 5.3.

Change-Id: I04f0e026f8a6e0414f08d5302aff6c084a1d19a7