linkat: avoid OS X 10.10 trailing slash with symlink bug
On Darwin 14.3.0 linkat(,"path1",,"dangling_symlink/",)
causes the symlink to be dereferenced, and if it points
to a non existent file, that file will be created as
a hard link to "path1".
This fixes a test failure in test-linkat.c.
* m4/linkat.m4 (gl_FUNC_LINKAT): Augment the test with
this case. The existing workaround in linkat.c for
trailing slash issues, suffices for this case.
* doc/posix-functions/linkat.texi: Add OS X 10.10 to
the list of platforms with trailing slash issues.