pgsql: Re-enable the old code in xlog.c that tried to use

Log Message:
-----------
Re-enable the old code in xlog.c that tried to use posix_fadvise(), so that
we can get some buildfarm feedback about whether that function is still
problematic.  (Note that the planned async-preread patch will not really
prove anything one way or the other in buildfarm testing, since it will
be inactive with default GUC settings.)

Modified Files:
--------------
    pgsql:
        configure (r1.625 -> r1.626)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/configure?r1=1.625&r2=1.626)
        configure.in (r1.584 -> r1.585)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/configure.in?r1=1.584&r2=1.585)
    pgsql/src/backend/access/transam:
        xlog.c (r1.326 -> r1.327)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.326&r2=1.327)
    pgsql/src/include:
        pg_config.h.in (r1.137 -> r1.138)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/pg_config.h.in?r1=1.137&r2=1.138)
        pg_config_manual.h (r1.35 -> r1.36)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/pg_config_manual.h?r1=1.35&r2=1.36)