pgsql: Allow background writing to be shut down by setting limit values

Log Message:
-----------
Allow background writing to be shut down by setting limit values to zero.
This does not disable the bgwriter process: it still has to wake up often
enough to collect fsync requests from backends in a timely fashion. But
it responds to the recent gripe about not being able to prevent the disk
from being spun up constantly.

Modified Files:
--------------
pgsql/doc/src/sgml:
runtime.sgml (r1.288 -> r1.289)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.288&r2=1.289)
pgsql/src/backend/storage/buffer:
bufmgr.c (r1.180 -> r1.181)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.180&r2=1.181)
pgsql/src/backend/utils/misc:
guc.c (r1.244 -> r1.245)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.244&r2=1.245)
postgresql.conf.sample (r1.132 -> r1.133)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.132&r2=1.133)