uqmi: Add proper IPv6 support

uqmi: Add proper IPv6 support

Use the new --ip-family option to start both IPv4 and IPv6 sessions
by default. Autoconnect can't be used when starting two sessions,
so revert back to using the client IDs and packet data handles for
handling the network connection.

Some modem firmwares do not implement a RA server, therefore by
default use outband IP configuration and static addressing. Some
other firmwares report bogus IP configuration with the WDS get
current settings command. In this case inband configuration with
DHCP/RA can be optionally enabled by setting option dhcp to 1.

Per 3GPP standard a /64 prefix is served to all clients, which is
extended to LAN as specified in RFC 7278.

v2: Restrict the IPv6 gateway route source address
Signed-off-by: Matti Laakso <malaakso@elisanet.fi>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46843 3c298f89-4303-0410-b956-a3cf2f4a3e73