pgsql: Modify interactions between sinval.c and sinvaladt.c.

Log Message:
-----------
Modify interactions between sinval.c and sinvaladt.c.  The code that actually
deals with the queue, including locking etc, is all in sinvaladt.c.  This means
that the struct definition of the queue, and the queue pointer, are now
internal "implementation details" inside sinvaladt.c.

Per my proposal dated 25-Jun-2007 and followup discussion.

Modified Files:
--------------
    pgsql/src/backend/storage/ipc:
        ipci.c (r1.94 -> r1.95)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/ipci.c?r1=1.94&r2=1.95)
        sinval.c (r1.83 -> r1.84)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/sinval.c?r1=1.83&r2=1.84)
        sinvaladt.c (r1.66 -> r1.67)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/sinvaladt.c?r1=1.66&r2=1.67)
    pgsql/src/backend/utils/init:
        postinit.c (r1.180 -> r1.181)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c?r1=1.180&r2=1.181)
    pgsql/src/include/storage:
        sinval.h (r1.46 -> r1.47)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/sinval.h?r1=1.46&r2=1.47)
        sinvaladt.h (r1.45 -> r1.46)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/sinvaladt.h?r1=1.45&r2=1.46)