Update :unused-fn-args warnings to be separate for each arg

Without these changes, the warnings generated no longer have a
line/col number or file name, due to recent changes in
tools.analyzer(.jvm).

Before these changes, the :unused-fn-args warnings were one combined
warning per argument vector, no matter how many arguments in that
vector were unused.  They also included the function name.

Now they no longer contain the function name, but I think it is better
to have a separate warning per unused argument, anyway.