Add =encoding utf8 to all core non-ASCII POD files + test script

Add =encoding utf8 to all core non-ASCII POD files + test script

It's now possible to run:

    perl Porting/checkpodencoding.pl

To check if the core contains any naughty POD that uses non-ASCII
without declaring an encoding.

With this patch all the POD in core (except POD tests we're ignoring)
has a correct =encoding directive. The script also flags problems in
lib/* and cpan/* which aren't being fixed as part of this commit.