tar: fix symlink race and symlink transform bug

tar: fix symlink race and symlink transform bug

Problem reported by Tobias Stoeckmann in:
http://lists.gnu.org/archive/html/bug-tar/2015-07/msg00004.html
* gnulib.modules: Add areadlinkat-with-size.
* src/create.c: Include areadlink.h.
(dump_file0): Use areadlinkat_with_size, rather than trying to do
it by hand, incorrectly.  This also avoids assumption that
the symlink contents fit on the stack.  Also, use the transformed
link name, not the original link name, when deciding whether the
name is long enough to require writing a long link.