posix-shell.m4: reject opensolaris's "sh (AT&T Research) 1993-12-28 s+" * m4/posix-shell.m4 (gl_POSIX_SHELL): Reject a shell that lets a umask modification leak out of a subshell. Otherwise, the opensolaris /bin/sh would be accepted and thus cause unwarranted failures in the coreutils test suite.