ln: replace destination links more atomically

If the file B already exists, commands like 'ln -f A B' and
'cp -fl A B' no longer remove B before creating the new link.
Instead, they arrange for the new link to replace B atomically.
This should fix a race condition reported by Mike Crowe (Bug#25680).
* NEWS, doc/coreutils.texi (cp invocation, ln invocation):
Document this.
* bootstrap.conf (gnulib_modules): Add symlinkat.
* src/copy.c, src/ln.c: Include force-link.h.
* src/copy.c (same_file_ok): It's also OK to remove a destination
symlink when creating symbolic links, or when the source and
destination are on the same file system and when creating hard links.
* src/copy.c (create_hard_link, copy_internal):
* src/ln.c (do_link):
Rewrite using force_linkat and force_symlinkat, to close a window
where the destination temporarily does not exist.
* src/cp.c (main): Do not set x.unlink_dest_before_opening
merely because we are in link-creation mode.
* src/force-link.c, src/force-link.h: New files.
* src/local.mk (copy_sources, src_ln_SOURCES): Add them.
* tests/cp/same-file.sh: Adjust test case to match fixed behavior.

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