[review] dcbw/icera-fixes: random Icera fixes and addition of IPv6 support

[Dan Williams]

Hi,

I dusted off WWAN IPv6 in NetworkManager again and found some
deficiencies in the Icera IPv6 support (eg, there was none).  Given that
the only non-Qualcomm modems I have that reliably support IPv6 are Icera
based, time to use it.

Note that the 21M only returns an IPv6 link-local address and a DNS
server, and the client should use normal IPv6 neighbor discovery
procedures (eg, RS and RA) to determine the IPv6 prefix and router.

There are also a couple other fixes to the Sierra Icera code based on
testing the Icera stuff.

Tested with Samsung Y3300, Sierra USB305, Nokia 21M, and ZTE/Vodafone
K3805-Z.  Please review!

Thanks,
Dan