pgsql: Use correct path separator for Windows builtin commands.

Use correct path separator for Windows builtin commands.

pg_upgrade produces a platform-specific script to remove the old
directory, but on Windows it has not been making sure that the
paths it writes as arguments for rmdir and del use the backslash
path separator, which will cause these scripts to fail.

The fix is backpatched to Release 9.0.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/f88ad86fba2a1700c96953ee26ed700f08fa5d7e

Modified Files
--------------
contrib/pg_upgrade/check.c      |   47 +++++++++++++++++++++++++++++++++-----
contrib/pg_upgrade/pg_upgrade.h |    2 +
2 files changed, 42 insertions(+), 7 deletions(-)