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_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/d10ddf4d510a5e366775f9a6bb20d0104ff1bcfd

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