O_DIRECT for WAL writes

This patch ticks off the following TODO items:
Consider use of open/fcntl(O_DIRECT) to minimize OS caching, especially for WAL writes.

The patch adds a new choice "open_direct" to wal_sync_method.
It uses O_DIRECT flags for WAL writes, like O_SYNC.

I had sent a patch looked like this before
(http://candle.pha.pa.us/mhonarc/patches2/msg00131.html)
but I found it is not always needed to write multiple pages in one write()
because most disks have writeback-cache. So, I left only O_DIRECT routines
in the patch and it impacts a present source code less.

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ITAGAKI Takahiro
NTT Cyber Space Laboratories