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