pgsql: Assorted fixes to make pg_upgrade build on MSVC.

Log Message:
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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.

Modified Files:
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pgsql/contrib/pg_upgrade:
check.c (r1.2 -> r1.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_upgrade/check.c?r1=1.2&r2=1.3)
exec.c (r1.1 -> r1.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_upgrade/exec.c?r1=1.1&r2=1.2)
page.c (r1.1 -> r1.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_upgrade/page.c?r1=1.1&r2=1.2)
pg_upgrade.h (r1.2 -> r1.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_upgrade/pg_upgrade.h?r1=1.2&r2=1.3)