plperl: enable UTF-8 support

Hello,

Here's a patch I added against plperl, originally against beta5, now
against rc1. It simply checks with GetDatabaseEncoding() if the current
database is in UTF-8, and if so, sets the UTF-8 flag on the arguments
that are passed to perl. This means that it isn't necessary to
utf8::upgrade() every string, as perl has no way of knowing offhand
that a string is UTF-8 -- but postgres does, because the database
encoding is specified, so it makes sense to turn the flag on. You
should also be able to properly manipulate UTF-8 strings now from
plperl as opposed to plperlu, because otherwise you'd have to use
encoding 'utf8' which was not allowed. It could also eliminate some
unexpected bugs if you assume that perl knows the string is unicode. It
is enabled only for perl 5.6 and higher, so earlier versions will not
be affected.

I have been assured by crab that the patch is quite harmless and will
not break anything. It would be great to see it in 8 final! :-)

Regards,
Dave