Re: ArchiveEntry optional arguments refactoring

Here is another version, where I accumulated all the suggestions:

* Use NULL as a default value where it was an empty string before (this
  required few minor changes for some part of the code outside ArchiveEntry)

* Rename defn, descr to createStmt, description

* Use a macro to avoid pgindent errors

About the last one. I'm also inclined to use the simpler version of
ARCHIVE_OPTS macro, mostly because the difference between "optional" and
"positional" arguments in the alternative proposal is not that visible. So

> mixing struct arguments and normal function arguments seems
> quite confusing

could probably affect not only readability, but also would be bit more
problematic for updating this code (which was the goal in the first place).