pgsql: Turn autovacuum on by default.

Log Message:
-----------
Turn autovacuum on by default.  (stats_row_level is also on by default.)
Threshold and scale factor are cut in half for more aggressive behavior.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.75 -> r1.76)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.75&r2=1.76)
        maintenance.sgml (r1.59 -> r1.60)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/maintenance.sgml.diff?r1=1.59&r2=1.60)
    pgsql/src/backend/utils/misc:
        guc.c (r1.343 -> r1.344)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.343&r2=1.344)
        postgresql.conf.sample (r1.187 -> r1.188)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.187&r2=1.188)