[udp] don't make socket spin if a network I/F is down

UDP treats ENETUNREACH as a temporary error and just retries,
but this causes the TX thread to spin just doing sendto() therefore
blocking all other traffic.

(To reproduce this try starting corosync with 2 links configured in
corosync.conf but only one of them configured to the 'right' address

• it will spin in a tight loop and need to be killed with -9)

SCTP does not seem to suffer from this.