pgsql: In pg_upgrade on Windows, check if the directory is writable by

In pg_upgrade on Windows, check if the directory is writable by actually
creating and removing a file because access() doesn't work on that
platform.

Backpatch to 9.1 where this check was added.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/081a5518c0a7dcccfc76a12ae9d593648b68ce53

Modified Files
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contrib/pg_upgrade/exec.c | 41 ++++++++++++++++++++++++++++++++---------
1 files changed, 32 insertions(+), 9 deletions(-)