asp: Allow setting IP address as SCTP primary upon conn establishment

This is an initial implementation to set the local SCTP Primary address
as well as the peer's Primary Address (through SCTP ASCONF messages).
The Primary address is only set upon conn establishment (after connect()
 for clients, or accept() for severs), which means no logic is
introduced to make sure that primary address is kept during the entire
operation of the SCTP association (for instance if the Primary address
becomes unavailable and the stack changes the primary, and later on that
address becomes available again, the primary addres won't be set back to
the initially configured one).

Related: OS#6076
Change-Id: I4a9fc1a4ad82ed20ece328bc53fca58299d744ca
---
M TODO-RELEASE
M include/osmocom/sigtran/osmo_ss7.h
M src/osmo_ss7.c
M src/osmo_ss7_vty.c
M tests/vty/ss7_asp_test.vty
5 files changed, 317 insertions(+), 31 deletions(-)