module: network: add 'type` tag

This tag maps to the ifinfomsg->ifi_type member, which is set to one
of the ARPHRD_xyz values, defined in linux/if_arp.h.

There's a *ton* of them, and we can't possibly add a string mapping
for _all_ of them, so for now, set to one of:

* loopback
* ether
* wlan
* ARPHRD_NNN, where N is a number

doc/yambar-modules-network.5.scd

@@ -21,6 +21,10 @@ address per network interface.
 |  name
 :  string
 :  Network interface name
+|  type
+:  string
+:  Interface type (*ether*, *wlan*, *loopback*, or *ARPHRD_NNN*, where
+   *N* is a number).
 |  index
 :  int
 :  Network interface index