pgsql: Rename standby_keep_segments to wal_keep_segments.

Log Message:
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Rename standby_keep_segments to wal_keep_segments.

Also, make the name of the GUC and the name of the backing variable match.
Alnong the way, clean up a couple of slight typographical errors in the
related docs.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.268 -> r1.269)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.268&r2=1.269)
        high-availability.sgml (r1.60 -> r1.61)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/high-availability.sgml?r1=1.60&r2=1.61)
    pgsql/src/backend/access/transam:
        xlog.c (r1.400 -> r1.401)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.400&r2=1.401)
    pgsql/src/backend/utils/misc:
        guc.c (r1.548 -> r1.549)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.548&r2=1.549)
        postgresql.conf.sample (r1.283 -> r1.284)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.283&r2=1.284)
    pgsql/src/include/access:
        xlog.h (r1.108 -> r1.109)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog.h?r1=1.108&r2=1.109)