pgsql: Make smart shutdown work in combination with Hot

Log Message:
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Make smart shutdown work in combination with Hot Standby/Streaming Replication.

At present, killing the startup process does not release any locks it holds,
so we must wait to stop the startup and walreceiver processes until all
read-only backends have exited.  Without this patch, the startup and
walreceiver processes never exit, so the server gets permanently stuck in
a half-shutdown state.

Fujii Masao, with review, docs, and comment adjustments by me.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        runtime.sgml (r1.433 -> r1.434)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml?r1=1.433&r2=1.434)
    pgsql/doc/src/sgml/ref:
        pg_ctl-ref.sgml (r1.49 -> r1.50)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_ctl-ref.sgml?r1=1.49&r2=1.50)
    pgsql/src/backend/postmaster:
        postmaster.c (r1.604 -> r1.605)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c?r1=1.604&r2=1.605)