pgsql: Add support for wal_fsync_writethrough for Darwin, and

Log Message:
-----------
Add support for wal_fsync_writethrough for Darwin, and restructure the
code to better handle writethrough.

Chris Campbell

Modified Files:
--------------
    pgsql/doc/src/sgml:
        runtime.sgml (r1.319 -> r1.320)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.319&r2=1.320)
    pgsql/src/backend/access/transam:
        xlog.c (r1.192 -> r1.193)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.192&r2=1.193)
    pgsql/src/backend/storage/file:
        fd.c (r1.115 -> r1.116)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/file/fd.c.diff?r1=1.115&r2=1.116)
    pgsql/src/include/access:
        xlog.h (r1.60 -> r1.61)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog.h.diff?r1=1.60&r2=1.61)
    pgsql/src/include/port:
        darwin.h (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port/darwin.h.diff?r1=1.6&r2=1.7)
        win32.h (r1.44 -> r1.45)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port/win32.h.diff?r1=1.44&r2=1.45)
    pgsql/src/include/storage:
        fd.h (r1.50 -> r1.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/fd.h.diff?r1=1.50&r2=1.51)