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>