pgsql: Make vacuum_defer_cleanup_age be PGC_SIGHUP level, since it's not

Log Message:
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Make vacuum_defer_cleanup_age be PGC_SIGHUP level, since it's not sensible
to have different values in different processes of the primary server.
Also put it into the "Streaming Replication" GUC category; it doesn't belong
in "Standby Servers" because you use it on the master not the standby.
In passing also correct guc.c's idea of wal_keep_segments' category.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.289 -> r1.290)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.289&r2=1.290)
    pgsql/src/backend/storage/ipc:
        procarray.c (r1.70 -> r1.71)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c?r1=1.70&r2=1.71)
    pgsql/src/backend/utils/misc:
        guc.c (r1.558 -> r1.559)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.558&r2=1.559)
        postgresql.conf.sample (r1.289 -> r1.290)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.289&r2=1.290)