Send a BVC-RESET to all persistent Gb interfaces at start-up

3GPP TS 48.018 Section 8.4:

> After any failure affecting the NSE, the party (BSS or SGSN) where
> the failure resided shall reset the signalling BVC. After sending or
> receiving a BVC-RESET PDU for the signalling BVC, the BSS shall stop all
> traffic and initiate the BVC-RESET procedure for all BVCs corresponding
> to PTP functional entities of the underlying network service entity. The
> BSS must complete the BVC-RESET procedure for signalling BVC before
> starting PTP BVC-RESET procedures.

TODO: We should not just trigger a single outbound BVC-RESET message,
but we should re-transmit them until we get a response.   This would
likely entail adding FSMs to libosmogb, which we will leave for a later
point - it's anticipated that the NS + BSSGP code is undergoing quite
some changes in the coming months anyway, so leave it for then.

Change-Id: I0b46035b40709c38bb9ab9493c11031a577e3ee0
Closes: OS#4629
Depends: libosmocore.git I353adc1aa72377f7d4b3336d2ff47791fb73d62c