noglob wip
archive: convert pathspec to 'struct pathspec' sooner

We store a "const char **" in our archiver_args, but then
convert it to a "struct pathspec" at point of use. Let's
just move to the final form immediately, which will open up
more room for refactoring pathspecs.
run_add_interactive: use argv_array

This makes the code a little simpler, as we do not have to
worry about pre-counting the pathspecs, nor about the magic
"5" to leave room for other arguments.