pgsql: Make directory name comparisons on Win32 case insensitive.

Log Message:
-----------
Make directory name comparisons on Win32 case insensitive.

This method will not catch all different ways since the locale
handling in NTFS doesn't provide an easy way to do that, but it
will hopefully solve the most common cases causing startup
problems when the backend is found in the system PATH.

Attempts to fix bug #4694.

Tags:
----
REL8_2_STABLE

Modified Files:
--------------
    pgsql/src/port:
        path.c (r1.70 -> r1.70.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/port/path.c?r1=1.70&r2=1.70.2.1)