pgsql: Fix pg_current_logfile() to not emit a carriage return on Window

Fix pg_current_logfile() to not emit a carriage return on Windows.

Due to not having our signals straight about CRLF vs. LF line
termination, the output of pg_current_logfile() included a trailing
\r on Windows. To fix, force the file descriptor it uses into text
mode.

While here, move a couple of local variable declarations to make
the function's logic clearer.

In v12 and v13, also back-patch the test added by 1c4e88e2f so that
this function has some test coverage. However, the 004_logrotate.pl
test script doesn't exist before v12, and it didn't seem worth adding
to older branches just for this.

Per report from Thomas Kellerer. Back-patch to v10 where this
function was added.

Discussion: https://postgr.es/m/412ae8da-76bb-640f-039a-f3513499e53d@gmx.net

Branch
------
REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/765ad260a18fc65003ef244b401ef83525ff3da4

Modified Files
--------------
src/backend/utils/adt/misc.c | 19 ++++++++++++-------
1 file changed, 12 insertions(+), 7 deletions(-)