Add support for systemd network interface names

From the documentation on http://freedesktop.org:

> Starting with v197 systemd/udev will automatically assign predictable, stable
> network interface names for all local Ethernet, WLAN, and WWAN interfaces.
> This is a departure from the traditional interface naming scheme (`eth0`,
> `eth1`, `wlan0`, ...), but should fix real problems.

The new scheme uses the following prefixes: `en*` for Ethernet, `wl*` for WLAN,
and `ww*` for WWAN. Therefore, simply using a prefix of `e*`, as is done in
this commit, should work fine for locating Ethernet interfaces in both the old
and new naming schemes.