pgsql: Allow ShowBufferUsage() to report the number of reads/writes that

Log Message:
-----------
Allow ShowBufferUsage() to report the number of reads/writes that have
occurred to temporary files.  This replaces the unused
NDirectFileRead/NDirectFileWrite counters.

Itagaki Takahiro

Modified Files:
--------------
    pgsql/src/backend/storage/buffer:
        buf_init.c (r1.80 -> r1.81)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/buf_init.c?r1=1.80&r2=1.81)
        bufmgr.c (r1.237 -> r1.238)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c?r1=1.237&r2=1.238)
    pgsql/src/backend/storage/file:
        buffile.c (r1.31 -> r1.32)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/file/buffile.c?r1=1.31&r2=1.32)
    pgsql/src/include/executor:
        execdebug.h (r1.32 -> r1.33)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/execdebug.h?r1=1.32&r2=1.33)
    pgsql/src/include/storage:
        buf_internals.h (r1.98 -> r1.99)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/buf_internals.h?r1=1.98&r2=1.99)