link,ln: use gnulib's link module to work around Solaris 10 deficiency

link,ln: use gnulib's link module to work around Solaris 10 deficiency

Before this change, :>f; ln -T f no-such/ would succeed on Solaris 10.
After it, ln fails, as it should: ln: accessing `z/': Not a directory
The command, link f no-such/, had the same problem on that system.
* bootstrap.conf (gnulib_modules): Add "link".
* tests/ln/slash-decorated-nonexistent-dest: New test.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Portability): Mention the improvement.