gprs_ns: Don't use initial IP/port for anything but SNS

Section 6.2.1 of 3GPP TS 48.016 states:
> A pre-configured endpoint shall not be used for NSE data or signalling
> traffic (with the exception of Size and Configuration procedures) unless
> it is configured by the SGSN using the auto-configuration procedures.

However, in the current SNS implementation, the initial IP/Port over
which we perform the SNS-SIZE + SNS-CONFIG are treated as one of the
normal NS-VCs. Specifically, we also perform the NS-ALIVE procedure on
it, which is clearly wrong.

Let's explicitly create the "initial" NS-VC with data and signalling
weight of 0, and ensure we never start the alive timer or send any
non-SNS PDUs on this connection as long as SNS was not used to change
either of the two weights to non-zero.

While at it, also safeguard against processing any incoming non-SNS
messages on such a all-zero-weight connection.

Change-Id: I16a91a07e5914d123b2ea2f8413b94e7cd518628
Closes: OS#3844

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