mwan3: add "use" function to mwan3 utils

mwan3: add "use" function to mwan3 utils

Use "mwan3 use" to wrap a command with interface bindings so that you can
avoid the mwan3 rules and test behavior on a specific interface.

eg "mwan3 use wan ping -c1 1.1.1.1"

Additional binding arguments to the command will have their system
calls intercepted and ignored.

eg "mwan3 use wan ping -c1 -I tun0 1.1.1.1" will use the
device associated with "wan", rather than "tun0".

Signed-off-by: Aaron Goodman <redacted>