Modules 825 mod_itk fix

ref: https://tickets.puppetlabs.com/browse/MODULES-825
This commit fix the wrong behaviour of the apache module in case of mod_itk enabled on apache 2.4.
prior to apache 2.4 mod_itk and mod_prefork were mutually exclusive, now itk work along prefork so some fail test in itk.pp and prefork.pp are no more needed if version is >= 2.4
On mpm.pp an if statement has been added in order to fix an anomaly with mod_itk library name on ubuntu ( and possibly all debian) distros. the current manifest try to compose library name using "mod_mpm_" prefix. this behaviour is correct many apache mods but not for mod_itk that installs "mpm_itk.so". this leads the generated itk.load  to load a nonexistant library file stopping apache service restart.
In order to preserve previous behaviour an if statement has been added to create a symlink to the real file in the debian case branch.