pgsql: Simplify and rename some GUC variables, per various recent

Log Message:
-----------
Simplify and rename some GUC variables, per various recent discussions:

* stats_start_collector goes away; we always start the collector process,
unless prevented by a problem with setting up the stats UDP socket.

* stats_reset_on_server_start goes away; it seems useless in view of the
availability of pg_stat_reset().

* stats_block_level and stats_row_level are merged into a single variable
"track_counts", which controls all reports sent to the collector process.

* stats_command_string is renamed to track_activities.

* log_autovacuum is renamed to log_autovacuum_min_duration to better reflect
its meaning.

The log_autovacuum change is not a compatibility issue since it didn't exist
before 8.3 anyway.  The other changes need to be release-noted.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.145 -> r1.146)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.145&r2=1.146)
        maintenance.sgml (r1.81 -> r1.82)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/maintenance.sgml?r1=1.81&r2=1.82)
        monitoring.sgml (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml?r1=1.52&r2=1.53)
        release.sgml (r1.507 -> r1.508)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml?r1=1.507&r2=1.508)
    pgsql/src/backend/commands:
        analyze.c (r1.108 -> r1.109)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/analyze.c?r1=1.108&r2=1.109)
        vacuumlazy.c (r1.98 -> r1.99)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.98&r2=1.99)
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.59 -> r1.60)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.59&r2=1.60)
        pgstat.c (r1.164 -> r1.165)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c?r1=1.164&r2=1.165)
    pgsql/src/backend/utils/misc:
        guc.c (r1.420 -> r1.421)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.420&r2=1.421)
        postgresql.conf.sample (r1.228 -> r1.229)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.228&r2=1.229)
    pgsql/src/include:
        pgstat.h (r1.66 -> r1.67)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h?r1=1.66&r2=1.67)
    pgsql/src/include/postmaster:
        autovacuum.h (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/postmaster/autovacuum.h?r1=1.11&r2=1.12)
    pgsql/src/test/regress/expected:
        stats.out (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/stats.out?r1=1.11&r2=1.12)
    pgsql/src/test/regress/sql:
        stats.sql (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/stats.sql?r1=1.9&r2=1.10)