readlink: detect FreeBSD bug readlink("link-to-symlink/",buf,len) mistakenly read the contents of "symlink", rather than failing. * m4/readlink.m4 (gl_FUNC_READLINK): Also detect FreeBSD bug with slash on symlink. * doc/posix-functions/readlink.texi (readlink): Document the bug. * tests/test-readlink.h (test_readlink): Enhance test. Signed-off-by: Eric Blake <ebb9@byu.net>