Update the Perl 5.6 and 5.8 installers for P4Perl.
These are installers
for P4Perl 3.4582. Since the 3.x version of P4Perl no longer requires the
P4-Client module, I've moved the installer scripts down a level. I've also
added some additional code in the installer to check the Perl version
being used matches (roughly) the version it was built for. This helps to stop
people (like me) with multiple Perl versions from accidentally breaking
things.