popen-safer: prevent popen from clobbering std descriptors

popen-safer: prevent popen from clobbering std descriptors

* modules/popen-safer: New file.
* lib/popen-safer.c: Likewise.
* m4/stdio-safer.m4 (gl_POPEN_SAFER): New macro.
* lib/stdio--.h (popen): Provide override.
* lib/stdio-safer.h (popen_safer): Provide declaration.
* tests/test-popen.c (includes): Partially test this.
* modules/popen-safer-tests: New file, for more tests.
* tests/test-popen-safer.c: Likewise.
* MODULES.html.sh (file stream based Input/Output): Mention it.

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