pgsql: Add pg_file_sync() to adminpack extension.

Add pg_file_sync() to adminpack extension.

This function allows us to fsync the specified file or directory.
It's useful, for example, when we want to sync the file that
pg_file_write() writes out or that COPY TO exports the data into,
for durability.

Author: Fujii Masao
Reviewed-By: Julien Rouhaud, Arthur Zakirov, Michael Paquier, Atsushi Torikoshi
Discussion: https://www.postgresql.org/message-id/CAHGQGwGY8uzZ_k8dHRoW1zDcy1Z7=5GQ+So4ZkVy2u=nLsk=hA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d694e0bb793ebd6b392e6ef6a3b0a59ae66cbc95

Modified Files
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contrib/adminpack/Makefile                |  3 ++-
contrib/adminpack/adminpack--2.0--2.1.sql | 17 +++++++++++++++++
contrib/adminpack/adminpack.c             | 25 +++++++++++++++++++++++++
contrib/adminpack/adminpack.control       |  2 +-
contrib/adminpack/expected/adminpack.out  | 17 +++++++++++++++++
contrib/adminpack/sql/adminpack.sql       |  6 ++++++
doc/src/sgml/adminpack.sgml               | 19 +++++++++++++++++++
src/backend/storage/file/fd.c             |  3 +--
src/include/storage/fd.h                  |  1 +
9 files changed, 89 insertions(+), 4 deletions(-)