Changed execution of tinc-up:

- Do not free() strings that have been putenv()d, see man page of the
  latter.
- Do not set IFNAME anymore, it appears that the ioctl to get the name of
  the interface does not work at all. Since it is set to NETNAME in case
  of tun/tap and it is known beforehand in case of ethertap, there is no
  need for it anyway... (though it would've simplified things).