freopen-safer: new module

freopen-safer: new module

* modules/freopen-safer: New module.
* m4/stdio-safer.m4 (gl_FREOPEN_SAFER): New macro.
* lib/freopen-safer.c (freopen_safer): New file.
* lib/stdio-safer.h (freopen_safer): New declaration.
* lib/stdio--.h (freopen): New override.
* MODULES.html.sh (File stream based Input/Output): Mention it.
* doc/posix-functions/freopen.texi (freopen): Mention pitfalls and
freopen-safer module.
* doc/posix-functions/stderr.texi (stderr): Likewise.
* doc/posix-functions/stdin.texi (stdin): Likewise.
* doc/posix-functions/stdout.texi (stdout): Likewise.
* modules/freopen-safer-tests: New test.
* tests/test-reopen-safer.c: New file.

Signed-off-by: Eric Blake <ebb9@byu.net>