pgsql: Clean up some code, comments and docs referring to Windows 2000

Clean up some code, comments and docs referring to Windows 2000 and older

This fixes and updates a couple of comments related to outdated Windows
versions.  Particularly, src/common/exec.c had a fallback implementation
to read a file's line from a pipe because stdin/stdout/stderr does not
exist in Windows 2000 that is removed to simplify src/common/ as there
are unlikely versions of Postgres running on such platforms.

Author: Michael Paquier
Reviewed-by: Kyotaro Horiguchi, Juan José Santamaría Flecha
Discussion: https://postgr.es/m/20191219021526.GC4202@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e2e02191e23379502a38a6b0436ab7f41b2efc08

Modified Files
--------------
doc/src/sgml/install-windows.sgml |   3 +-
doc/src/sgml/installation.sgml    |   2 +-
src/backend/libpq/auth.c          |   6 +-
src/bin/initdb/initdb.c           |  11 ++--
src/common/exec.c                 | 132 +-------------------------------------
src/port/getaddrinfo.c            |  12 +---
6 files changed, 13 insertions(+), 153 deletions(-)