unix_filename_rubout deletes to the previous slash or whitespace,
unlike the previously implemented backwards-kill-word which treats and
non-alphanumeric character as a boundary.
To illustrate, given the text 'foo/bar.baz', unix_filename_rubout will
delete 'bar.baz' while backwards-kill-word will delete only 'baz'.
See #1710.
This restores the previous behavior of `unix-word-rubout` as
`backward-kill-word`, which is closer to the naming used in readline.
It is bound to <Alt-Backspace> by default, though <Ctrl-Backspace> will
also work due to a builtin binding.
Resolves#1698.
