settle-dad.sh expects netmask to be provided, but netmask is not marked 'required'

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Description: Ubuntu Xenial Xerus (development branch)
Release: 16.04

ifupdown:
  Installed: 0.8.10ubuntu1
  Candidate: 0.8.10ubuntu1
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For the inet6 static method, interfaces(5) implies that only 'address' is required (and thus netmask is optional), however settle-dad.sh assumes IF_NETMASK is being set. If only address is supplied, the following message is logged:

Waiting for DAD... Error: inet6 prefix is expected rather than "2001:db8:ab:cd::12/"

and settle-dad.sh returns immediately, before DAD has settled.

'ip address list' appears to ignore the netmask if it is supplied, so settle-dad could probably just use '[..] to "${IF_ADDRESS}"'. Alternatively, 'netmask' could be documented as 'required'.