pgsql: The attached patch changes units of the some default values in

Log Message:
-----------
The attached patch changes units of the some default values in
postgresql.conf.

- shared_buffers = 32000kB => 32MB
- temp_buffers = 8000kB => 8MB
- wal_buffers = 8 => 64kB

The code of initdb was a bit modified to write MB-unit values.
Values greater than 8000kB are rounded out to MB.

GUC_UNIT_XBLOCKS is added for wal_buffers. It is like GUC_UNIT_BLOCKS,
but uses XLOG_BLCKSZ instead of BLCKSZ.

Also, I cleaned up the test of GUC_UNIT_* flags in preparation to
add more unit flags in less bits.

ITAGAKI Takahiro

Modified Files:
--------------
pgsql/src/backend/utils/misc:
guc.c (r1.352 -> r1.353)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.352&r2=1.353)
postgresql.conf.sample (r1.195 -> r1.196)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.195&r2=1.196)
pgsql/src/bin/initdb:
initdb.c (r1.122 -> r1.123)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/initdb/initdb.c.diff?r1=1.122&r2=1.123)
pgsql/src/include/utils:
guc_tables.h (r1.28 -> r1.29)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc_tables.h.diff?r1=1.28&r2=1.29)