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
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/4397c51816cbb918fc3aebc474dd53bc231502b9

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