pgsql: Ensure that all details of the ARC algorithm are hidden within

Log Message:
-----------
Ensure that all details of the ARC algorithm are hidden within freelist.c.
This refactoring does not change any algorithms or data structures, just
remove visibility of the ARC datastructures from other source files.

Modified Files:
--------------
    pgsql/src/backend/storage/buffer:
        buf_init.c (r1.70 -> r1.71)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/buf_init.c.diff?r1=1.70&r2=1.71)
        buf_table.c (r1.38 -> r1.39)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/buf_table.c.diff?r1=1.38&r2=1.39)
        freelist.c (r1.49 -> r1.50)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/freelist.c.diff?r1=1.49&r2=1.50)
    pgsql/src/include/storage:
        buf_internals.h (r1.75 -> r1.76)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/buf_internals.h.diff?r1=1.75&r2=1.76)