pgsql: Allow concurrent-safe open() and fopen() in frontend code for Wi

Allow concurrent-safe open() and fopen() in frontend code for Windows

PostgreSQL uses a custom wrapper for open() and fopen() which is
concurrent-safe, allowing multiple processes to open and work on the
same file. This has a couple of advantages:
- pg_test_fsync does not handle O_DSYNC correctly otherwise, leading to
false claims that disks are unsafe.
- TAP tests can run into race conditions when a postmaster and pg_ctl
open postmaster.pid, fixing some random failures in the buildfam.

pg_upgrade is one frontend tool using workarounds to bypass file locking
issues with the log files it generates, however the interactions with
pg_ctl are proving to be tedious to get rid of, so this is left for
later.

Author: Laurenz Albe
Reviewed-by: Michael Paquier, Kuntal Ghosh
Discussion: https://postgr.es/m/1527846213.2475.31.camel@cybertec.at
Discussion: https://postgr.es/m/16922.1520722108@sss.pgh.pa.us

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/0ba06e0bfb8cfd24ff17aca92aa72245ddd6c4d7

Modified Files
--------------
src/bin/initdb/initdb.c | 8 ++++++++
src/bin/pg_basebackup/pg_receivewal.c | 2 +-
src/bin/pg_verify_checksums/pg_verify_checksums.c | 2 +-
src/common/file_utils.c | 4 ++--
src/include/port.h | 3 ---
5 files changed, 12 insertions(+), 7 deletions(-)