Support syncing WAL log to disk using either fsync(), fdatasync(),

Support syncing WAL log to disk using either fsync(), fdatasync(),
O_SYNC, or O_DSYNC (as available on a given platform).  Add GUC parameter
to control sync method.
Also, add defense to XLogWrite to prevent it from going nuts if passed
a target write position that's past the end of the buffers so far filled
by XLogInsert.