Patch to add hook to copydir()

Hello all,

I've been working on an extension that tightly integrates
postgres with underlying filesystem . I need to customize
how postgres copies directories for new databases.

I first looked at the ProcessUtility_hook. This would require
me to copy or rewrite most of the createdb() function. This is
less than ideal of course. Someone on the IRC channel
suggested I could add a hook for copydir().

I implemented the hook similar to how the
ProcessUtility_hook is implemented. I couldn't find any tests
for any of the existing hooks. I've been looking at the regression
tests, but I am not entirely sure how to proceed on that front.

I tested my patch extensively against master and
the REL_12_STABLE branch. All tests pass and the patch has
been working great with my extension.

I attached a first draft of the patch against master.

---
Swen Kooij