popen: Set gl_cv_func_popen_works to "guessing no" on OS/2. * doc/posix-functions/popen.texi: List OS/2 kLIBC case. * m4/popen.m4 (gl_FUNC_POPEN): Set gl_cv_func_popen_works to "guessing no" on OS/2. Copyright-paperwork-exempt: Yes