pgsql: Enable configurable log of autovacuum actions.

Log Message:
-----------
Enable configurable log of autovacuum actions. Initial patch from Simon
Riggs, additional code and docs by me. Per discussion.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.120 -> r1.121)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.120&r2=1.121)
pgsql/src/backend/commands:
analyze.c (r1.104 -> r1.105)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/analyze.c.diff?r1=1.104&r2=1.105)
vacuumlazy.c (r1.85 -> r1.86)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c.diff?r1=1.85&r2=1.86)
pgsql/src/backend/postmaster:
autovacuum.c (r1.41 -> r1.42)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c.diff?r1=1.41&r2=1.42)
pgsql/src/backend/utils/misc:
guc.c (r1.385 -> r1.386)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.385&r2=1.386)
postgresql.conf.sample (r1.214 -> r1.215)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.214&r2=1.215)
pgsql/src/include/postmaster:
autovacuum.h (r1.9 -> r1.10)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/postmaster/autovacuum.h.diff?r1=1.9&r2=1.10)