Add support for systemd network interface names

From the documentation on 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.