Changes

gprs_ns2_sns: move the NS-ALIVE PDU of a new NS-VC after the SNS-ACK

When adding a new NS-VC after the SNS-CONFIG is done, the new NS-VC would send out a
NS-ALIVE before sending out the SNS-ACK towards the remote NSE.
This isn't a big issue, because the SNS-ACK can be reordered by the network anyways,
but be nice and ensure the order matches the expectation.

Related: OS#6611
Change-Id: I6503bbb32b4440eb53f8c9bd03572f4a189f1a86