Linux: Support wireless IP roaming This is achieved by checking that the interface is wireless, IFF_UP and IFF_LOWER_UP are present, but IFF_RUNNING is missing. This gives exactly the same support as modern NetBSD when carrier loss is detected, but without the address verifications when the carrier comes back as that needs to be handled in the kernel. While IP setup is maintained, other configuration data is discarded. Note that this should be improved in the future. Thanks to Boris Krasnovskiy <borkra@gmail.com> for helping with this.

--- a/src/if.h	Wed Dec 09 11:15:30 2020 +0000
+++ b/src/if.h	Sat Dec 12 13:12:26 2020 +0000
@@ -161,6 +161,7 @@
 #define if_getmtu(ifp) if_domtu((ifp), 0)
 #define if_setmtu(ifp, mtu) if_domtu((ifp), (mtu))
 int if_carrier(struct interface *, const void *);
+bool if_roaming(struct interface *);
 
 #ifdef ALIAS_ADDR
 int if_makealias(char *, size_t, const char *, int);