Support direct I/O on macOS.

Support direct I/O on macOS.

Macs don't understand O_DIRECT, but they can disable caching with a
separate fcntl() call.  Extend the file opening functions in fd.c to
handle this for us if the caller passes in PG_O_DIRECT.

For now, this affects only WAL data and even then only if you set:

  max_wal_senders=0
  wal_level=minimal

This is not expected to be very useful on its own, but later proposed
patches will make greater use of direct I/O, and it'll be useful for
testing if developers on Macs can see the effects.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BADiyyHe0cun2wfT%2BSVnFVqNYPxoO6J9zcZkVO7%2BNGig%40mail.gmail.com