pgsql: Marginal code cleanup for streaming replication.

Log Message:
-----------
Marginal code cleanup for streaming replication.

There is no reason that proc.c should have to get involved in this dirty hack
for letting the postmaster know which children are walsenders.  Revert that
file to the way it was, and confine the kluge to pmsignal.c and postmaster.c.

Modified Files:
--------------
    pgsql/src/backend/storage/ipc:
        pmsignal.c (r1.30 -> r1.31)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/pmsignal.c?r1=1.30&r2=1.31)
    pgsql/src/backend/storage/lmgr:
        proc.c (r1.221 -> r1.222)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/proc.c?r1=1.221&r2=1.222)
    pgsql/src/include/storage:
        pmsignal.h (r1.32 -> r1.33)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/pmsignal.h?r1=1.32&r2=1.33)