sometimes forwarding fails with EACCES

[christf]

mmfd is running, sees neighbours, forwards a packet on 2 mesh interfaces. One forward works, the other sendmsg is returned with "Permission denied".

Wed Jan 15 20:18:49 2020 daemon.err mmfd[2174]: Forwarding packet from fe80::183b:50ff:fe6e:f7d9 with destaddr=ff02::1234, nonce=0x67d9a28c86778053 to fe80::1%mesh-vpn-1 [17].
Wed Jan 15 20:18:49 2020 daemon.err mmfd[2174]: Forwarding packet from fe80::183b:50ff:fe6e:f7d9 with destaddr=ff02::1234, nonce=0x67d9a28c86778053 to fe80::9%mesh-vpn-0 [18].
Wed Jan 15 20:18:49 2020 daemon.err mmfd[2174]: sendmsg on interface mesh-vpn-0 (fe80::9): Permission denied

According to the manpage this can happen

(For UDP sockets) An attempt was made to send to a network/broadcast address as though it was a unicast address.

According to the logs, this is not what is attempted here. The error condition happens in https://github.com/freifunk-gluon/mmfd/blob/master/src/main.c#L169