getcwd: Work around getcwd bug on AIX 5..7.

getcwd: Work around getcwd bug on AIX 5..7.

* m4/getcwd-path-max.m4 (gl_FUNC_GETCWD_PATH_MAX): Require
AC_CANONICAL_HOST. Assign exit code 31 to the bug seen on AIX 5.1..7.1.
Use a different value for gl_cv_func_getcwd_path_max. Move the
definition of HAVE_PARTLY_WORKING_GETCWD from here...
* m4/getcwd.m4 (gl_FUNC_GETCWD): ... to here. Invoke
gl_FUNC_GETCWD_PATH_MAX also when $gl_cv_func_getcwd_null is 'no'.
Define HAVE_MINIMALLY_WORKING_GETCWD.
* lib/getcwd.c (__getcwd): Don't use the system's getcwd on platforms
where it is not even minimally working, that is, on AIX.
* tests/test-getcwd.c (test_long_name): Distinguish the same cases as
m4/getcwd-path-max.m4.
(main): Update exit code computation.
* doc/posix-functions/getcwd.texi: Mention list of platforms where
getcwd does not handle long file names.