pgsql: 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(at)anarazel(dot)de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BADiyyHe0cun2wfT%2BSVnFVqNYPxoO6J9zcZkVO7%2BNGig%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2dbe8905711ba09a2214b6e835f8f0c2c4981cb3

Modified Files
--------------
src/backend/storage/file/fd.c | 36 +++++++++++++++++++++++++++++++++++
src/bin/pg_test_fsync/pg_test_fsync.c | 35 ++++++++++++++++++++++++++++++----
src/include/access/xlogdefs.h | 15 ---------------
src/include/storage/fd.h | 16 ++++++++++++++++
4 files changed, 83 insertions(+), 19 deletions(-)