rm: output number of arguments at the interactive prompt

rm: output number of arguments at the interactive prompt

Include the number of arguments which rm received in the "Remove all
arguments?" prompt.  This is useful in the, presumably, common case
where the arguments were not provided by hand, but instead were the
result of various shell expansions.  A simple, if somewhat contrived,
example (assuming rm is aliased to rm -I) could be:

  rm * .o

where the prompt "Remove 120 arguments?" is more likely to make
the user catch the problem.

* src/rm.c (main): Include correctly pluralized n_files
in the output message.  Also remove the now redudant "all".
* tests/rm/interactive-always.sh: Adjust to the new prompt.
* tests/rm/interactive-once.sh: Likewise.