pgsql: Fix platform-specific test for path prefix-ness: move it into

Log Message:
-----------
Fix platform-specific test for path prefix-ness: move it into path.c where
it can be done right.  Allow explicit use of absolute DataDir path.
Per Dave Page.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        genfile.c (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/genfile.c.diff?r1=1.5&r2=1.6)
    pgsql/src/include:
        port.h (r1.81 -> r1.82)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port.h.diff?r1=1.81&r2=1.82)
    pgsql/src/port:
        path.c (r1.57 -> r1.58)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/path.c.diff?r1=1.57&r2=1.58)