pgsql: Fix fsync code to test whether F_FULLFSYNC is available, instead

Log Message:
-----------
Fix fsync code to test whether F_FULLFSYNC is available, instead of
assuming it always is on Darwin.  Per report from Neil Brandt.

Modified Files:
--------------
    pgsql:
        configure (r1.472 -> r1.473)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.472&r2=1.473)
        configure.in (r1.442 -> r1.443)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.442&r2=1.443)
    pgsql/src/backend/storage/file:
        fd.c (r1.124 -> r1.125)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/file/fd.c.diff?r1=1.124&r2=1.125)
    pgsql/src/include:
        pg_config.h.in (r1.89 -> r1.90)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pg_config.h.in.diff?r1=1.89&r2=1.90)
    pgsql/src/include/port:
        darwin.h (r1.8 -> r1.9)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port/darwin.h.diff?r1=1.8&r2=1.9)