abis: Avoid TCP/IPA RSL sockets continue conn establishment while shutting down

when something fails in osmo-bts during startup and it goes into SHUTDOWN state,
it is desirable to close new RSL links (sockets) which are in progress to connect,
while it waits for a while to complete shutdown (power ramping down, etc.).
This way we don't end up with new interactions and new state against a BSC if we
are shutting down.

The new libosmo-abis API is used since the higher layer struct e1inp_sign_link assigned
to each struct gsm_bts_trx is not provided to the osmo-bts code layer by libosmo-abis API
until the TCP+IPA handshake in the socket becomes fully established (sign_link_up()
callback).

Depends: libosmo-abis.git Ia6418321f3b6f1f7274efd414625a4b10a09a362
Change-Id: I599d074f51f490b43c9a89b105d1823391926947
---
M TODO-RELEASE
M src/common/abis.c
2 files changed, 9 insertions(+), 0 deletions(-)