wireproxy needs very little permissions, we can restrict it to basically
nothing. DynamicUser means the system will generate a UID on demand
for service, also CAP_NET_BIND_SERVICE can be used to allow this user
to bind to a port < 1024 if desired. Also LoadCredential lets us read
a file with tight permissions i.e. root:root 0400 and pass it to only
wireproxy in an ephemeral and constrained manner.
Signed-off-by: J. Dekker <jdek@itanimul.li>