simplify: one g_hnbgw as global state and root ctx

So far we jump through hoops everywhere to pass around osmo-hnbgw's
global singleton state to all code paths. Some files choose to spawn a
static "global" pointer. And we also have the talloc root tall_hnb_ctx.

Simplify:
- Have a single global g_hnbgw pointer. Drop all function args and
  backpointers to the global state.
- Use that global g_hnbgw as talloc root context, instead of passing a
  separate tall_hnb_ctx around.

(Cosmetic preparation for separate osmo_hnbgw_main.c file)

Change-Id: I3d54a5bb30c16a990c6def2a0ed207f60a95ef5d
---
M include/osmocom/hnbgw/context_map.h
M include/osmocom/hnbgw/hnbgw.h
M include/osmocom/hnbgw/hnbgw_cn.h
M include/osmocom/hnbgw/hnbgw_pfcp.h
M include/osmocom/hnbgw/ps_rab_fsm.h
M src/osmo-hnbgw/context_map.c
M src/osmo-hnbgw/context_map_rua.c
M src/osmo-hnbgw/context_map_sccp.c
M src/osmo-hnbgw/hnbgw.c
M src/osmo-hnbgw/hnbgw_cn.c
M src/osmo-hnbgw/hnbgw_hnbap.c
M src/osmo-hnbgw/hnbgw_pfcp.c
M src/osmo-hnbgw/hnbgw_rua.c
M src/osmo-hnbgw/hnbgw_vty.c
M src/osmo-hnbgw/mgw_fsm.c
M src/osmo-hnbgw/ps_rab_ass_fsm.c
M src/osmo-hnbgw/ps_rab_fsm.c
17 files changed, 234 insertions(+), 255 deletions(-)