pgsql: Fix WIN32 wait() return value macros to be accurate, particularly

Log Message:
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Fix WIN32 wait() return value macros to be accurate, particularly
because they are used for testing the return value from system().
(WIN32 doesn't overlay the return code with other failure conditions
like Unix does, so they are just simple macros.)

Fix regression checks to properly handle diff failures on Win32 using
the new macros.

Modified Files:
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    pgsql/src/include/port:
        win32.h (r1.53 -> r1.54)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port/win32.h.diff?r1=1.53&r2=1.54)
    pgsql/src/test/regress:
        pg_regress.c (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/pg_regress.c.diff?r1=1.16&r2=1.17)