setenv, unsetenv: work around various bugs

setenv, unsetenv: work around various bugs

POSIX requires setenv(NULL,"",0), setenv("a=b","",0),
unsetenv(NULL), and unsetenv("a=b") to fail with EINVAL, but
many BSD implementations lack validation.  The gnulib
replacement for void unsetenv did not do validation, and the
replacement for setenv was out of sync with glibc.  Also, some
BSD implementations of setenv("a","==",1) eat the leading '='.

See also some recent Austin Group interpretations on environ:
http://austingroupbugs.net/view.php?id=167
http://austingroupbugs.net/view.php?id=185

* lib/setenv.c (setenv) [!HAVE_SETENV]: Resync from glibc.
(setenv) [HAVE_SETENV]: Work around bugs.
* lib/unsetenv.c (unsetenv) [HAVE_UNSETENV]: Work around bugs.
* m4/setenv.m4 (gl_FUNC_SETENV_SEPARATE, gl_FUNC_UNSETENV): Check
for bugs.
(gl_FUNC_SETENV): Write in terms of gl_FUNC_SETENV_SEPARATE.
* m4/environ.m4 (gl_ENVIRON): Avoid expand-before-require.
* m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Update defaults.
* modules/stdlib (Makefile.am): Update substitutions.
* lib/stdlib.in.h (setenv, unsetenv): Update prototypes.
* doc/posix-functions/setenv.texi (setenv): Document the bugs.
* doc/posix-functions/unsetenv.texi (unsetenv): Likewise.
* modules/setenv-tests: New test.
* modules/unsetenv-tests: Likewise.
* tests/test-setenv.c: New file.
* tests/test-unsetenv.c: Likewise.

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