pgsql: Add prefetching support on macOS

Add prefetching support on macOS

macOS doesn't have posix_fadvise(), but fcntl() with the F_RDADVISE
command does the same thing.

Some related documentation has been generalized to not mention
posix_advise() specifically anymore.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/0827edec-1317-4917-a186-035eb1e3241d%40eisentraut.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/6654bb92047b37cee053cedd6fa1829841b2ad8e

Modified Files
--------------
doc/src/sgml/config.sgml        | 14 ++++------
doc/src/sgml/wal.sgml           |  4 +--
src/backend/commands/variable.c |  4 +--
src/backend/storage/file/fd.c   | 57 +++++++++++++++++++++++++++--------------
src/include/pg_config_manual.h  |  5 ++--
src/include/port/darwin.h       |  5 ++++
6 files changed, 54 insertions(+), 35 deletions(-)