Add support for Linux TUN/TAP devices

[johnny-mnemonic]

• configure.ac: check for Linux TUN/TAP support
• src/netdev.c: rearrange, fix typos, add support for Linux TUN/TAP devices

Original patch from @mikpe and available from: https://user.it.uu.se/~mpe22148/linux/patches/ia64/ski-1.3.2/ski-1.3.2-tuntap.patch

Adapted for Ski post version 1.5.0

---

• configure.ac: keep "normal" Ski networking by default
• src/netdev.c: make TAP networking work beyond Ski's host system
• README-networking.md: Add README for networking options

[johnny-mnemonic]

This patch set allows a Ski instance to be networked similar to how it was done with QEMU in the past. I.e.:

• a host interface for network connectivity beyond the host
• one TAP device per simulation/emulation instance
• a bridge connecting these interfaces at the data link layer.

It requires no additional physical network card like the older BPF based networking* but still offers the same functionality as that one. This will be especially useful for hosting multiple Ski instances on the same host, as it only requires virtual interfaces for the Ski instances. In addition a networked Ski instance can now run as ordinary user, if the used TAP device was pre-created before launching Ski.

*) At least I never found a way to make the BPF based networking work w/o an extra host interface for the Ski instance.

The support for TUN/TAP devices is not enabled by default, but only with --enable-tuntap during the configure step. A user can only select one (BPF based networking) or the other (TUN/TAP networking) currently.

Also an extra README covering the networking options was added.

[trofi]

Looks good! Merged as is.