readlink: document portability issue with symlink length

readlink: document portability issue with symlink length

Per comments in areadlink, ERANGE on a too-small buffer is
expected on some platforms; making the readlink module guarantee
GNU behavior of truncated contents is counter-productive, since
we would be duplicating areadlink to learn a-priori how large to
make the buffer, and since truncated contents are not as useful.

* doc/posix-functions/lstat.texi (lstat): Mention that some file
systems have bogus st_size on symlinks, and mention the
areadlink-with-size module.
* doc/posix-functions/fstatat.texi (fstatat): Likewise.
* doc/posix-functions/readlink.texi (readlink): Mention the
areadlink module, and ERANGE failure.
* doc/posix-functions/readlinkat.texi (readlinkat): Likewise.
* tests/test-readlink.c (main): Relax test for AIX, HP-UX.

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