mwan3: "use" action: run process via `exec` and handle whitespace

mwan3: "use" action: run process via `exec` and handle whitespace

Previously the "use" command had the following shortcomings:
* a subprocess was created instead of replacing the shell process
* whitespace in arguments was not handled correctly

Implementation detail:
In shell context the `"$@"` expression should be used (instead of `$*`).
This allows the safe handling of arguments containing whitespace.

Closes: #20001
Signed-off-by: Lars Kruse <devel@sumpfralle.de>