native: create tap interfaces ad-hoc

PR currently only works in Linux, I will add support for other systems once the concept is ACK'd.

Usage: ignore `tapsetup.sh`, just use `make term` and get as many tap interfaces as you need.

TODO:
- [x] don't create unless term target is given
- [ ] integrate support for OSX/FreeBSD
- [ ] update documentation
OT:
While there's probably many ways to break this, aliasing is not one of them ;-)
An alias only exists in the running shell, the command executed is still the one the alias specifies.