mobile: add MNCC handler selection to settings

Since the mobile application is potentially able to maintain
multiple MS instances, it's better to have a possibility to
choose an MNCC (Call Control) handler per each MS separately.

This change removes the command-line option '-m', which was used
for enabling the external MNCC.  Now it's possible configure the
MNCC handler for each MS via the VTY interface and settings.

The following MNCC-handlers are available:

  - internal - built-in MNCC-handler (default);
  - external - external MNCC-handler via UNIX-socket (e.g. LCR);
  - dummy - dummy handler without CC support.

Change-Id: I2df91c7a79ba5c39bc6ceae900ef649129dd0346
Related: OS#3400
---
M doc/examples/mobile/default.cfg
M doc/examples/mobile/multi_ms.cfg
M src/host/layer23/include/osmocom/bb/mobile/app_mobile.h
M src/host/layer23/include/osmocom/bb/mobile/settings.h
M src/host/layer23/src/mobile/app_mobile.c
M src/host/layer23/src/mobile/main.c
M src/host/layer23/src/mobile/mnccms.c
M src/host/layer23/src/mobile/settings.c
M src/host/layer23/src/mobile/vty_interface.c
9 files changed, 112 insertions(+), 36 deletions(-)