networkd: simplify and display all devtypes

networkd: simplify and display all devtypes

Every place the kernel actually calls SET_NETDEV_DEVTYPE, it's adding a
piece of information that looks useful and relevant for us to use. So
let's use it when it's there.

The previous matching based on the corresponding ARPHRD didn't really
make much sense. The more sensible logic for getting a textual
representation of the link type is to see if the kernel supplies a
devtype. If it does, great. If not, then we can fall back on the ARPHRD,
as before.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>