use partial strings for passing argv to toplevel.pl

use partial strings for passing argv to toplevel.pl

In run_top_level: rename variables, make them partial strings

The rename is mostly cosmetic: all of argv is passed, and if argv
contains flags, goals, etc, it's more than filenames.

Using partial strings instead of atom came from discussions about how to
pass goals in the CLI, and can thus be considered preliminary work for
that.

In toplevel.pl: convert partial string to atom before passing it to
`use_module`.

Signed-off-by: Stephan Renatus <[email protected]>