Create extension infrastructure for the core procedural languages.

This mostly just involves creating control, install, and
update-from-unpackaged scripts for them.  However, I had to adjust plperl
and plpython to not share the same support functions between variants,
because we can't put the same function into multiple extensions.

catversion bump forced due to new contents of pg_pltemplate, and because
initdb now installs plpgsql as an extension not a bare language.

Add support for regression testing these as extensions not bare
languages.

Fix a couple of other issues that popped up while testing this: my initial
hack at pg_dump binary-upgrade support didn't work right, and we don't want
an extra schema permissions test after all.

Documentation changes still to come, but I'm committing now to see
whether the MSVC build scripts need work (likely they do).