rm improvements from next rebased and pulled onto master

[Jim Meyering]

Remember those changes on "next" to make rm more efficient in some
pathological cases.  I've just tidied up NEWS, moving its content into
the two main commits, and then pulled that series onto master.
Here's post-7.6 NEWS so far:

  ** Improvements

    rm: rewrite to use gnulib's fts
    This makes rm -rf significantly faster (400-500%) in some pathological
    cases, and slightly slower (20%) in at least one pathological case.

    rm -r deletes deep hierarchies more efficiently.  Before, it took O(N^2)
    time, now it takes O(N).  However, this improvement is not as pronounced
    as might be expected for very deep trees, because prior to this change, for
    any relative name length longer than 8KiB, rm -r would sacrifice official
    conformance to avoid the disproportionate O(N^2) performance penalty.
    Leading to another improvement:

    rm -r is now slightly more standards-conformant when operating on
    write-protected relative file names longer than 8KiB.

This is big enough that I'm making a snapshot already, for the adventurous.
I'll post about that separately.