Merge branch 'perldelta' into blead

Merge branch 'perldelta' into blead

Add a new mode to Porting/corelist-perldelta.pl to update the module
changes in pod/perldelta.pod for the perl you are currently building.

This can be run as:

./perl -Ilib Porting/corelist-perldelta.pl --mode=update pod/perldelta.pod

*after* updating Module::CoreList with something like:

./perl -Ilib Porting/corelist.pl cpan

There is still scope for improvement in the handling of removed modules,
and in automatically cribbing information from the Changes files of those
modules that have been updated, but this is a big step in the right
direction.