Exclude call_user_func() and call_user_func_array()

The documentations for xhprof notes that the call graphs are much easier to read when these two functions are omitted — and it's correct. Simplifies things nicely.

To do this, in xhprof.module, around line 101, I replaced the call to xhprof_enable() thusly:

    xhprof_enable($flags,
      array('ignored_functions' =>  array('call_user_func',
        'call_user_func_array')));

It would be excellent if this were an option via the UI.