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

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This fixes and updates a couple of comments related to outdated Windows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