pcu_sock: forward downlink IMMEDAITE ASSIGNMENT with no IMSI to AGCH

When the SAPI PCU_IF_SAPI_PCH_DT is used, messages should include an
IMSI. However, there may be situations in the PCU where no IMSI is
available when sending an IMMEDIATE ASSIGNMENT through the PCH. In those
cases we cannot calculate the paging group. Even though it would work to
use the paging group 0, it is more appropriate to send the IMMEDIATE
ASSIGNMENT through the AGCH then.

As a background information to this it should be noted that this works
because the IMSI is commonly not known during a GMM ATTACH REQUEST. In
this phase the MS is in non-DRX mode, which means that it listens on
all CCCH blocks (PCH and AGCH)

See also: 3gpp TS 44.060, section 5.5.1.5 and
          3gpp TS 45.002, section 6.5.3, 6.5.6

Related: OS#6097
Change-Id: Iaa3e5cdcf86578821e149b3e23482a0bcee19f7b
---
M include/osmo-bts/bts.h
M src/common/bts.c
M src/common/pcu_sock.c
3 files changed, 69 insertions(+), 4 deletions(-)