Assorted fixes to make pg_upgrade build on MSVC.

Assorted fixes to make pg_upgrade build on MSVC.

* There is no chmod() on Windows.
* Must always use the 3-parameter version of open()
* There is no dynloader.h - but it also appears unnecessary on all platforms
* Don't include shlobj.h because it causes compile errors, and from what I can
  see it's not actually used. This may need to be added back for mingw
  and/or cygwin in the worst case.