ifplugd: if SIOCSIFFLAGS fails with EADDRNOTAVAIL, don't die

When using ifplugd on a RNDIS interface with monitor mode, using
SIOCSIFFLAGS seems to fail with EADDRNOTAVAIL. Treat it the same
as if iface disappeared again.

This has been observed on a Tegra TK1 using Linux 4.14. There seem
to be a race when the kernel is creating the device:
  ifplugd(usb0): started: BusyBox v1.24.1 (2018-03-27 09:47:48 CEST)
  ifplugd(usb0): interface doesn't exist, waiting
  ifplugd(usb0): interface appeared
  ifplugd(usb0): upping interface
  ifplugd(usb0): setting interface flags failed: Cannot assign requested address
  <exit>

With this patch ifplugd is successful in upping the device the second
time around:

  ifplugd(usb0): upping interface
  ifplugd(usb0): using SIOCETHTOOL detection mode
  ifplugd(usb0): interface appeared
  ifplugd(usb0): upping interface
  ifplugd(usb0): setting interface flags failed: Cannot assign requested address
  ifplugd(usb0): interface appeared
  ifplugd(usb0): upping interface
  ifplugd(usb0): using SIOCETHTOOL detection mode
  ifplugd(usb0): link is up
  ifplugd(usb0): executing '/etc/ifplugd/ifplugd.usb.action usb0 up'
  ifplugd up
  ifplugd(usb0): exit code: 0

function                                             old     new   delta
up_iface                                             120     127      +7

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

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